Use eye contact as a means to start flirting with a guy before either of you ever open your mouth. If you notice that a guy is looking in your direction, look up so that you make eye contact for a moment and then look back in the direction you were just looking in. If the guy isn't looking your way, then move around a little or turn your head towards him once or twice but don't overdo it.
Smile lightly while staying a little bit quiet at first both before you've started talking to the guy and just as you begin conversation. The smile encourages the guy to keep going and the little bit of silence and shyness allows you to keep an air of mystery, making the guy even more eager to flirt and find out what's going on beneath the surface.
Laugh or giggle during your conversation even if what the guy's saying isn't that funny. Dating experts often use this tip as a great way to get rid of some of the butterflies and make your conversation a lot more natural and flowing.
Engage more as the conversation goes on and really emphasize things that you find you have in common with the guy. Change your body position a few times during the conversation so that you alternately lean forward and then back and then even touch the guy with your fingertips after he makes a joke or if you want to pretend to tell him something secretive that requires you to lean towards him.
Keep the ending of the flirt session conservative and cool by flashing your silent smile and agreeing that you'll see the guy around. If the guy doesn't ask for your number or to go out, don't sweat it since he'll probably be back later after he realizes how well the flirting went.