Hold his eyes till he turns away, also blush if you can
The ONLY way it'll work, ...u MUST find a way to BREAK OUT of your SHYNESS & start a conversation with him
They probably like you anyway unless you reject them . Situations aren't perfect all the time ,right ? So you make a couple mistakes or you're awkward . There's a little leeway .And it depends what you want or what you're after . If it's just innocent flirting ( can't speak for myself , lousy flirter ) than you can learn as you go . but if you're looking for a boyfriend just be friendly and be yourself. I have such a hard time getting girls that it always amazes me when some girls admit to being awkward and shy too , I always assume girls are just born naturally confident in social situations
Look on the bright side. At least you can smile! I either give them a weird look, or look away immediately. Sometimes, I can't even tell if they're checking me out or not. But, to answer your question, try giving a flirtatious wink, and waving hello.
You have to stop caring what people think about you of you make a move. Just have fun with it :) life's to short to care about stuff like that