Well it would be a good start to be friends and get to know each other, see if he likes you. When your prepared to ask him it’s better to do it in private.
If you get an inside joke going between just the two of you, work it into conversation often.
Say their name when you're talking to them. (It's true! People thrill at the sound of their own name—especially when uttered by someone they're interested in.)
Find a common enemy: another party guest, an annoying guy at the bar, a broken jukebox, the lack of pizza joints in this part of town. It's you two against the world.
If they mention an ex, or a date gone bad, tell them that they're too good for that person, anyway.
Give them a ridiculously huge tip. (This only works if they're your waiter, bartender or barista. Obviously.)
Email them because you just saw something and it "made me think of you."
Treat them to something sometime.
Make up a nickname for them. Be the only one who calls them that.
Invent any excuse for them to come over — a new movie, a sports game, a dinner party, a home repair project you really need help with.
Be genuinely delighted every time you see them — make no effort to hide it.