“There’s a guy I like at work, and I think he might like me, although he already has a girlfriend. We’re friends, and I think we flirt a bit (I know I do), but I’m wondering how I might take it to the next level. Any suggestions?”
By take it to the next level, I’m assuming you mean “get in his pants.”
What you need to do is insure his attraction to you while maintaining your approachability.
My suggestion would be to continue spreading the charm – verbal banter, playful flirting, etc. – but every once in a while, take it a little too far.
Get him laughing, then look him in the eyes a little too long afterward, as if you just realized you could love him (or lay him across the copy machine, as it were).
Then say something like, “Oh, sorry, I forgot you have a girlfriend.” Giggle.
Leave him wondering what he’s doing with her and not you.