Cross your legs: The more you cross and uncross your legs in front of a guy, the more interested you are. I believe they teach this to men in pre-school.
Go for the one-two punch: Want to really see him sweat? Cross your legs while holding eye contact and turn toward him. Next, let your high-heeled shoe drop until it's dangling from your toe. Then begin to slowly and seductively move your foot up and down in a thrusting (get the picture?) motion. Note, this only works with high, sexy heels. The effect is dampened somewhat if you attempt it with sneakers or sensible flatties. (Sorry, but I didn't promise comfort along with flirtability!)
Teach with touch: Once he's talking to you, stroke your collarbone absentmindedly with the ring finger of the opposite hand. Maintain eye contact with him as you're doing it, and let your hand rest there when you've finished. (If stroking doesn't feel natural, just leave your hand there. Splay your fingers and lift your hand so just the tips of your fingers touch.)
Turn up the heat: Now throw your head back slightly. (It will look the most natural if you wait until the next time you laugh.) Move your hand to the base of your throat. Now ever so slowly, let your fingers slide down your throat toward your breasts. Stop just where your cleavage starts and let your fingers rest there. Keep them there as long as he talks but when you talk, remove your hand. Can you think of a better incentive for him to keep talking to you?