7 Things To Put On A Flyer/Poster/Ad

Get at least one other person, who can double-check the details, to proof-read it. It is very common to have typos, and not unknown to get the dates wrong (having the convention from Thursday to Saturday instead of Friday to Sunday by looking at the wrong year on the calendar when checking the dates). Misspelling your Guests names is not a good idea (I very nearly sent out a set of ads for a major convention with our internationally famous Guest of Honour's name misspelled! It was only because I happened to show it to someone else and they said, "Are you sure that's how <insert name of guest here!> spells his name?" and of course it wasn't!)

(in no particular order)

The name of the convention
The dates of the convention (in a format that will be understood internationally. Don't put 04-08-98 since to the US that is the eighth of April 1998 and to Europeans it is the fourth of August. It will be worse in a few years when 01/02/03 could be the 1st of February 2003, the 2nd of January 2003 or even the 3rd of February 2001!)
The location of the convention. If you are allowing people to join "at the door", make sure that the name/location of the hotel/convention centre/student union or whatever site you're using is precise enough so that people will be able to find it
The contact details for the convention (Address, Phone, Fax, E-mail, Web, etc.) Make sure it is obvious which address is the contact address and which address is the location of the convention! (Not a common error, but worth double-checking)
The guests (if any)
The type of convention (e.g. Science Fiction, Star Trek, X-Files, Humour in Science Fiction)
The membership rates (if these change often, you can put in a line saying "please write for details" instead. See Tips below)

Major programme items that will draw people. E.g. Magic-the Gathering World Tournament, Writing for Star Trek workshop, Paul Darrow and William Shatner performing Shakespeare...
Other things at the con or in the area that may draw people. E.g. just one mile from Disneyland, world firework competition outside the hotel on Saturday night, total eclipse of the sun (indoors if wet), free beer from the brewery next door...
Pretty pictures/graphics. There are a *lot* of conventions, and unless you're only distributing your flyers directly to your intended audience, you need to catch the eye of the people who will end up joining your convention. If you're running a Doctor Who convention, put a large piece of text or a large graphic saying DOCTOR WHO (perhaps the diamond shaped logo) on your flyer (both sides if it is double-sided, because it may be the wrong side up when a potential member glances at it!)
Colour. Colour printing is still expensive, but using a bright/unique colour for the paper for your flyer is a cheap alternative and will help it stand out from all the other flyers
The names of the committee. This is only useful if a) they are a reasonably well-known bunch of people (to that particular fandom) and have a reputation for running good conventions, or b) if they have enough friends that will look at the poster and say "golly, Bob Smith is on the committee. I guess I'll have to join since he's a good friend and I should support him in his endeavours!" (this second reason does happen more often than you just thought!)
The Theme. If you are running a Star Wars convention, but decide that the special emphasis will be on the fact that most of the bad guys wear masks and you can see the faces of most of the good guys (yeah, I know R2D2 is a major exception to this theory...) then you could put your theme on your flyer since that may intrigue some people and cause them to join up because it sounds interesting to them.
The charities that the profits will be going to. There are some professional conventions where the profits go into the pockets of the organisers, but in general I would advise you to avoid them. The only exception I will make is that professional conventions usually charge more and so can afford guests that fan-run, not-for-profit conventions may not have the money to book. If you must see <insert name of expensive guest here, probably from Star Trek - The Original Series, and not Sulu, Chekov or many of the minor characters all of whom are willing to give some of their time, relatively inexpensively when the profits are going to charity...> then you will need to go to a professional con. Otherwise they are sucker-traps designed to get you in, and empty your wallets onto the dealers' tables and then get you out again, hopefully without annoying you enough so that you'll come back next time. One of the professional convention organisers in the UK decided that he didn't like having to pay tax on his profits so he has been reporting the not-for-profit conventions to the tax man. When I can get some concrete proof on this, I will be glad to advise you to never go to any of his events again, since all he has managed to do so far is deprive charities of much-needed money and caused at least two conventions to fold because they would not have been able to afford the tax bill a "professional" convention must pay.

If you are putting a tear-off form/slip on your flyer, make sure that the address/dates/etc for the convention are on the piece the person retains (the information can be on the tear-off slip as well, but once it has been posted off, that won't be any help to the attendee). If you are printing double-sided flyers make sure the information on the back side of the tear-off slip is not important!
I'm going to move these details to another page soon!
If you are designing an ad to fit in both UK and US publications (e.g. programme books, progress reports etc.) then remember that the standard paper sizes are different. In the UK a full-size sheet of paper is A4 (210mm by 297mm which is about 8 1/4 inches by 11 2/3 inches) while US paper is Letter (8 1/2 inches by 11 inches, or 216mm by 279mm approx). If you use the smaller of each dimension (210mm by 279 mm, or 8 1/4 inches by 11 inches) as your paper size and set your margins inside that, then your master ad can be used on both sides of the Atlantic. Western Europe uses A4 as a standard paper size, but I have no idea what size paper is used throughout the rest of the world. Also note that US paper when punched for a ring binder has three holes, one in the middle of the long edge and the other two approx one quarter of the page down and one quarter of the page up. European binders have two or four holes spaced 8cm apart (the two holes are punched 4cm abover and below the middle of the long edge, for four holes the additional holes are punched 8cm above/below the other two holes. 8cm is also the distance between the two holes in a high-density 3.5inch floppy disk so you can put such a disk straight into a European binder (if you write-protect it!))
It is quite common to have an ad or flyer that is half the size of the standard page (so you get two flyers on each printed/photocopied page and can then cut it in half to get two flyers) On European A4 paper, the next size down is A5 which is A4 folded in half to give about 148.5mm by 210mm. Because of the ratio of the lengths (1 : square root of 2) and a peculiar property of the square root of two (1/sqr(2) = sqr(2)/2) this means that the ratio of width to height remains the same each time you reduce the size by folding it in half (or increase it by putting two sheets together) so you can just use the % reduction/enlargement on the photocopier to put A4 images onto A5. A3 is twice A4, A2 is twice A3 etc. A0 is a sheet of paper which is 1 square meter, but with the sides in the same 1:sqr(2) ratio so it is about 840mm by 1188mm (there is another set of paper sizes called "B" where the B0 paper size is 1 metre on the short side and sqr(2) (1.414) metres on the long side and it goes down through B1 etc. by folding each larger page in half/quarters/eighths etc. B5 is the only "B" sized paper I have actually heard of, and I'm not aware of anyone who commonly uses it.
Therefore if you are designing an ad for use on European paper sizes, you can just design it once and enlarge/reduce it as necessary (with the obvious caveat that you need to be able to read the small type!) But US paper sizes don't work the same way so if you reduce a Letter sized flyer to put in a programme book that is Letter folded in half in size, it won't fit properly. (The long dimension is longer, in proportion to the shorter edge) So you must be more aware of what size ad/flyer you are producing if it is going into a US publication.
One of the most common layouts for a double-sided flyer is to have the large logo, convention name and the rest of the 7 "musts" above on one side, and to have a much wordier reverse side to give all the details you think will get people to join.
If you are laying out your flyer/poster/ad on a computer with 50 million fonts, and all sorts of neat graduated fill patterns and three-dimensional special effects, remember two things.
1) Victorian Music Hall posters used dozens of fonts, and unless you are deliberately trying to look like that for a special effect, it is much better to stick to a small number of fonts, weights and sizes (e.g. maybe two different typefaces (one for headings and one for body text) in two or three sizes (heirarchy of headings perhaps), with perhaps bold used for emphasis, if necessary)
2) How are you going (or someone else) going to print these flyers/posters/ads? If it is photocopy (as quite a lot is nowadays) then complex fonts, graduated fills, light drop shadows, and pretty colours on the screen are going to reproduce as muddy clumps on paper. If you're not sure how it will look, print it off and fax it to yourself (or photocopy a photocopy of a photocopy of the ad) and see how it looks. Your master print from your expensive postscript laser, if you don't plan ahead, by the time it is copied and perhaps the size changed to fit the programme book page and that pasted-up copy is photocopied for the masters for the print run which is then copied to produce the programme book, will look like absolute rubbish!
It is better to have far too many copies of a flyer than not enough
Flyers and posters are useless unless they are where the appropriate people are going to see them. Make sure that you get plenty to all the relevant conventions, book shops, fan clubs and organisations, local libraries and schools, university SF groups etc. and have the flyers in or distributed with the publications for other similar conventions (you can usually do a reciprocal deal where you send out their flyers in your publications in exchange so it will cost you little or nothing in money terms and may increase goodwill between your groups thereby getting you more members, volunteers and advice)
If your convention membership rates rise as time goes by (e.g. 30 pounds until one year before the convention, 35 pounds until six months before the convention, 40 pounds until one month before the convention and then 50 pounds at the door) then make it clear on the flyer/poster/ad what the current rate is, and if you can, when the rate changes and what it will be so that your flyers don't go out of date and have to be thrown away and new ones printed with the new rate. Of course you may not now what the new rates will be so you will have to put in a sentence like "30 pounds until December 31st 1997, send a letter to the address below for the updated rates after that"

Don't put the name of every author and TV star you've invited but haven't heard back from
Dozens of addresses for each department at your convention
Copyright material if you haven't either a) received permission to do so or b) confidence that use of a copyrighted logo/photograph will be ok because it is good publicity for the show/company in question and they haven't objected in the past (or c) a good lawyer!)
Anything libellous, pornographic, racist or otherwise objectionable. This is supposed to be good publicity! If you a doing a humourous convention and have a flyer saying "Join our convention or we shoot another puppy" and have a picture of a cute puppy with big soulfull eyes then that is probably just about ok, having a picture of a dead puppy is just sick and you need treatment. (NOT a real example!)
Anything ridiculing another fandom. While it might be funny to put on your flyer "Sad? Lonely? No friends? No social life? Unable to wash? Then join The Plan-9 Fanclub! If you don't fit the previous description then A-Team fandom wants YOU!" (again, not a real example) We each have our own interests and hobbies, but from outside SF fandom in general, we are all regarded as Star Trek fans and UFO nuts and the last thing we would want to do is to give them more ammunition to deride and disparage Science Fiction in general and our own sub-genres in particular.
"Free beer for everyone!" (even if it is true!) because a) you get a crowd of non-fans descending on your con just to drink all the beer, get drunk and cause trouble and b) there are laws about serving alcohol to minors and, depending on which country you're in, there may be other legal problems with licencing or with providing alcohol to someone who is already inebriated. I know that your people would be very careful about this, but it only takes one mistake and you can end up legally liable.
Kill Inigo Montoya's Father (well, you'd probably guessed that one already, but I couldn't think of a seventh thing not to do) Hang on... I know. The 7th thing not to do in your PR is to say that there will be a list of 7 things when you can only think of six!