As a guideline, try to make your prompt a sentence or short phrase, but if you have a longer scenario that's burning a hole in your hard drive and you're just bursting to offer it up in all it's glory, we'll take those too.
Feel free to suggest a particular rare pairing within a fandom if you think the prompt suits them, but bear in mind that something fairly open would allow more people to play since we cover so many fandoms here (something we love about the place!). Crossover rare pairs are acceptable too, so it's okay to suggest those when prompting if you have a brilliant idea for a particular one, or think your prompt lends itself to a crossover specifically (even if you're open to any pairing/crossover).
Finally, if you're set on it being a particular genre and/or rating, pop that in your comment as well (and I'm screening comments so they'll be a bit of a surprise when they're unveiled). This includes whether it should be open, or gen/het/slash/femslash if you have strong feelings about that - although remember this is a 'pairing' challenge (it doesn't have to include sex, but the attachment should probably feature some suggestion of togetherness - impending or actual - at the very least!)
And that's all the rules for now! Once the prompts are in and Bingo is done with, we'll be kicking things off!