DON'T Post ads for the same game more than once. Wait for the ad to drop off the lists or delete your old announcement before reposting.
DON'T Post to multiple categories. Choose the narrowest one that fits your game, and post your announcement there.
DON'T Use HTML in your ad. Show your skills on your own site, not this one.
DON'T Write a novel for an ad. If you have more information to impart than will fit within 3000 character limit, put it in a text file and email it to your applicants.
DO Fill out all fields of the form.
DO Leave the "http://" in the "Site" field, or your link will not render properly.
DO Use the text field to describe your game and to entice people to visit your site. Tell them the things they want to know: how many players you are looking for, if the game is new or established, whether it's open- or closed-ended, what the expected posting rates are, etc.
DO Pay attention to the date your ad will be discarded so you'll know when to come back and re-post it. It will appear at the top of the screen when your ad is accepted.
DO Tell me if there is a category you don't see that you simply must have. There's an email link at the bottom of every page. I think we've covered everything, but the fact that I may have missed something is not outside the realm of possibility.