Jeez, what is it with so many guys who are bad at flirting? Flirting is an art that has to be practiced. The more you practice, the better you will probably be.
Modestly I am not bad at it, but my partner could charm the balls off a brass monkey. I have a friend who is incredible at it, he can walk into anyplace you can think of - he just exhudes confidence - and a half hour later he will inevitably walk out with the person he has targeted.
Flirting behavior is even well documented; It can consist of conversation, body language, brief physical contact, strong eye contact, casual touches: such as gently touching a man's arm during conversation, smiling suggestively, flattery, etc
One way to flirt is to communicate playfulness or irony. Subtle double entendres, with one meaning more formally appropriate and another more suggestive is cool.
If you are really bad at flirting, or understanding the signals I have a suggestion for you:
Go to a pickup bar with the biggest dawg you know and sit back and watch how he flirts, look for the clues he gives off, and incorporate them in your own behaviour...
Truthfully though the biggest turn on for most men is a little bit of confidence, attitude, even cockiness...
Think of it as being a salesperson, you are selling yourself; if you are not confident on your product, why would anyone want to buy it?
If you sit there looking glum and 'poor, poor, pity poor me' then why should anyone want to approach you when they are looking for a bit of fun?