Here's what I do for my RP requests as of late (for 1x1s, at least). I tend to have pretty good luck finding RP partners, or at least, I like to feel I do:
1) I used to try to have "one main idea thread," but this hasn't been working out for me. I have had far better luck taking time out to detail out an idea and make an individual thread for it, with the thread title being a unique title for the idea. For example, if I wanted a story about a powerful fire spirit who has married a princess for the kingdom's prosperity, I might title my thread "The Ifrit's Bride." Unique titles often get attention. If I have space, I will include the gender pairing I want. In the body of the message, note whether you have a preference about writer gender or not. When I had a dedicated request thread, people did not want to sift through all those ideas. I suspect I've gotten some pretty awesome RP partners this way who would not have otherwise given me a second glance.
2) Pictures. Safe-for-work ones. I find that these are inspiring without being intimidating. I don't tend to read threads with too many links to explicit sexual images, and I've found more often than not too many images is just as bad as no images. I will still read a thread that contains one or two NSFW ones, but any more than that, and I am no longer interested. Use pictures tastefully and sparingly. One or two is about right. Up to four is probably okay. Any more than that, and you run the risk of over-saturating your thread with images.
3) Always have a link to your O/Os in your request thread, even if you have a link in your signature. Note whether people can post in reply to your thread or whether they should PM you instead. Make sure your O/O thread is detailed, too; I've found people prefer more detail rather than less in O/O threads.
4) Mention what post length you like in your posts and your partner's posts, if any. Mention how often you want replies.
5) Don't detail ideas too heavily. The more detailed/niche it is, the less likely you are to get a bite. But if you have too few details, you'll have the same problem from the opposite direction. I've found about three to four paragraphs is often the "magic" number, giving enough detail to interest people, but not so much that they feel they can't contribute to the idea.