I love flirting, 99.9% of the time I don't really mean anything by it, I'm just a natural at it. Most times I know them, and that's why I'm a good flirt. I tease people, crack jokes, make for interesting conversations. Now, in bars, RARELY do I find that perfect kind of guy that I want to approach, but if I did, it would start off with eye contact.
Catch the guys eyes for five whole seconds. It's aggravating, because to be truthful, I used to be HORRIBLE at eye contact, but mentally count it down (I got over the whole eye contact thing while going to the mall and doing it to random strangers). I have very expressive eyes, so usually the guy can get the fact that yes, I'm interested in him. SMILE!! (if your eyes can't do it, make your face do it) <---probably the most important thing (practice in the mirror if you have to). You've got to look friendly and approachable. If the guy doesn't walk up to you then, he's probably married or whatever... move on, but nothing ventured, nothing gained.
If you REALLY want this guy, (and Nocker's right, some guys need to be hit over the head with it). Go up to him, say hi (best pick up line) introduce yourself.. go with small talk (which I hate). If you're at the club..say wow, busy night.. how are you.. stuff like that. You don't have to go overboard like "You're the hottest guy in the place" (I have used this, oddly enough it worked..haha, but to my credit, I was really drunk, and he was really hot, but I don't recommend this method, because it's laying it on WAY too thick). BUT you can tell them that they seem like an interesting/friendly person. People like to hear that stuff, especially since they did catch your eye, and you did think that.
Make it fun.. practice always, you're not going to get good at something without doing it. Don't make it so "all or nothing", otherwise you're putting way too much into the first initial contact, and it becomes work. Leave that at the office.
There, some food for thought to the American women on this site. Don't just think.. do it!