Just tossing my unsolicited two centd in the ring here..
First, have an idea of the type of thread you want to write and of the sort of things you are interested in.
Second, have a look at the roleplay requests for the vender you want to write with. Look for people who have similar writing interests, or who have proposals that you find intriguing.
Third, read a few RPs by those people if you can (click their name. And on the new page click "find all threads by...". Or something like that. I din't remember the exact wording.) It's a way to see if you like their style.
Fourth, send that person a PM and sell them on the idea of playing with you. Propose a plot based on their ideas, or expand on their plot and show them you're willing to help make things fun. Most importantly, show them that you read their requests. And don't just send a PM that essentially says "hay baby wnna cyber kthnx lol asl?"
Fifth, be willing to work with your prospective partner. You may exchange multiple PMs with someone as you hash out the details, but planning (like foreplay) pays off big in the end.
Sixth, don't get discouraged. And keep at it.
I hope that helped.