Fortunately enough for me, I rarely have to ask. While I'm usually the one to initiate sex, my boyfriend will go down on me at the drop of a hat.
In response to the question though, experience tells me that girls respond to subtlety while guys respond to blunt requests and visual prompts. For example, I have sat with my legs spread waiting for my boyfriend to come home and when he walked in the door I said "Your face, here, now!" That works for most guys (yes, even the timid ones). For girls however it can require more finesse... Unless of course you yourself are a girl in which case the same approach will work.
Brit