Well, depends on how do you want them to look at you.
If you want them to have wet dreams about you and think about your fuckability just wear tight shirts, short dresses and bend over a lot. :P
If you want them to be attracted to your mind/perosnality just try to, as the second poster said, calm down and talk to them normally, if you hear them talking about some subject that you know of, just voice your opinion.
If you don't know the person you like, it'd be good if you at least once cought (is this the right word?) an eye contact with them and then smile. It'll help them understand that you like them and whether they're attracted to you or not, guys still like attention so they will think about you and there's chance they'll consider you as a possible girlfriend.
Generally, I think it's good to let the guy know you're attracted to them (not by telling, tho!), if they like you (or could like you) the same way, good. If not, at least you didn't spend too much time chasing after them.
