Here's a smart, single guy's opinion:

Subtle signs: Girls are 10 times better reading subtle signs than guys. Instead of trying so hard to give the right signs, just try to read signs off of guys so you know what he's thinking.
Confidence: If you know what the guy is thinking, it will give you a lot of confidence. I would only go after a girl who is confident. Just by being confident you will give the right signs too.
Dressing good: Matters a LOT. Someone who dresses good vs. average is the difference of a guy going over to you or just letting the opportunity pass. Plus, dressing good will boost your confidence, which is also good.

I'm a smart guy, if I find a girl who is as smart as me, and has confidence, and looks, there is no way I'm not gonna try to get that girl.
If the guy is not smart it could be opposite.
But what do you want, just any guy, or a smart guy?


And don't assume a guy is not into you.

This is how a guy's brain works, step by step: At every step a girl can be eliminated from a guy's mind. But if she passes the step, the girl stays on the guy's mind.
1: Sees a girl.
2: Checks her out. (Only looks matter at this stage. A lot of girls are gone by here)
3: Looks at if she's approachable (Best if she's alone, but 1 girl with her will work too if the guy is very confident)
4: Tries to start a conversation (If you are assuming the guy is into you at this point, you'll have a better chance of keeping his interest)
And if you think you don't know if the guy is interested or not, it's waaaay worse for the guy to know if the girl is really interested or not. If you are interested, and the guy is about to leave without asking for your number, the best thing you can do is write your number on a piece of paper and give it to him.