In my mind there are two types of flirting, used for different purposes. Being a gay male, I tend to 'flirt' with my female friends quite often. Usually this consists of outrageous flattery and teasing which (hopefully) nobody takes seriously but has fun with.
The second kind of flirting, for me, is the more serious flirtations. These usually consist of trying to make eye contact and smiling to attract attention, talking while keeping eye contact, moving slightly into his personal space and perhaps touching him lightly if the mood and conversation seem to lend itself to this. This type of flirting is a lot more difficult because there's a bit more at stake, and being a moderately shy person around people I feel attracted to, I tend to stammer or not be able to think of anything to say.
Flirting can be a lot of fun, it's mostly a game... think of it that way and perhaps it will make things easier.