I think about what people see first. The thread-title is the first thing they notice, which is always the most difficult part of attracting interest. I use pictures, so that's the second thing that they will see. The picture has to correlate well with the thread-title; if the thread-title is "The Arkham Horror," a picture if a rainbow unicorn will throw people off. After that, they see whatever is written first, which should be a [b]concise[/b] premise/plot summary. Much like what Brovo said, only I prefer when it is written out of character. Then, I usually add some information about things and bits, and a few casual rules ([url=http://www.roleplayerguild.com/topics/28697/posts/ooc]example[/url]).

If my roleplay is an original idea, I usually mask it with fandom. True story, I conducted an experiment on a roleplaying site once, where I posted the same idea, but with two different names. The first name was my own, and the second name was 'Final Fantasy.' The post with my own name for the roleplay generated 1 person, whereas the post with 'Final Fantasy' generated over 20. Yes.... this was exactly the same post. So, it's a cool trick.