Flirting isn't obvious. If you're up-front about your desire to date or have sex with someone, you're not flirting. In a flirtatious interaction, the excitement comes from the hint at a possibility.
For many people, the thrill of flirting is simply receiving attention. To get that attention, you have to give it. Use nonverbal signals -- eye contact, standing just a little closer than normal, facing the person when he or she is speaking. There's no one magic pick-up line. Rather, it's up to you to figure out what kind of conversation the other person might enjoy. "Wow, I like your shirt" is perfectly OK in the right context, and much better than a line that sounds rehearsed.
Next, wait for a response before sending the next signal. If the person backs away, frowns, folds arms or starts flirting with someone else, don't make things worse by following him/her around. You've got your answer.
Flirting can be a great way to figure out whether you should ask someone out. If one or both of you is married and flirting, keep things strictly G-rated. Words and looks are probably OK; touching is not. If you feel like you can't help yourself, then you're in too deep and need to spend less time with this person.
