Like a girlfriend? She'll probably just do it if you ask directly, mine have.
As for a random girl, well that takes a little finesse. Part of it is seeming like you really don't even care that much. She'll want to prove herself to you ("why isn't he turned on by me? I know I can turn him on..."). The girls on this site are inundated with creepy teenage boys randomly asking for cyber sex and naked pics so they're probably pretty impervious to those requests. Although, I've got to admit I've received some interesting messages from ladies in the last month or so. They're out there. Just don't act like a sleaze bag and they'll come to you.
i'd give mine to any guy who were to ask ha ha :P as long as they weren't like 80... cause that's just grosss. I especially like seeing the different reactions I get from guys :P Like, one guy called me "angel" and a few others said "wow!" and stuff like that. ha ha :) and usually if the guy is hot, I like to get ones back ;P
You can either find a girl that seems to be really sexual from the get-go. Or you can just be cool with a girl and not talk about sex, let her get comfortable with you, then bring it up as if you would to a girl in person about sex. Don't push it like a horndog and you might find that she could be more comfortable with you and into the idea. But yeah, don't push it on anyone.