**** Buddy

How would *you* go about asking someone if they wanted to be one?

You would lie and say the usual crap.

Instead of saying:
- Look you're not that attractive or interesting but you're shaggable and to be honest I wouldn't mind a bit of sex but I'd hate to be seen as your boyfriend. I wouldn't even dream of having anything serious with you.

You say:
- I really like you, you're such a fantastic person. The thing is, I'm not in the right frame of mind for a relationship so think we shouldn't make this serious.


And of course, if the girl was amazingly hot and your dreamgirl you would immediately beg her to be your girlfriend but in this case, she clearly isn't.

First, make sure they're the kind of girl/boy who is almost certain to say yes. Otherwise you might risk insulting them/hurting their feelings etc. If you're not sure, and think they're either not interested, or they want more than just sex i.e. a relationship, chances are that even if they agree, it'll get messy.
I've been "propositioned" in this area recently, and he basically just said "I need a good **** buddy", and all I had to do was go "hey, I'm up for it", or whatever. It was over MSN though... so less courage needed for asking, and less cringe-y for rejection.