If a like a guy, i normally show it quite subtly, touching his arm gently, or looking at him, then looking away as soon as his eyes meet mine. Well it worked with my boyfriend anyway!
And it's interesting to note that, irrespective as to whether the outcome be positive or negative, such transactions are equally likely to be concluded with her foot in your crotch.
When I like a guy I try to hide it by not showing it at all. But sometimes if he talks to me I might tense up or stutter a bit. I also can't help but staring at him, but I'm tricky to catch. I never show it because I'm afraid of what he'll think.