Well, how old you are makes a difference. Teenage boys won't always ask you out because dating is too new to them, and if they do they probably have too MUCH experience.
Take it from me, I'm 16 and almost seven months pregnant: you don't want to worry too much about dating at all. It's more trouble than it's worth.
There's more to life then having a boyfriend or girlfriend, breaking up and then switching again. Doing so is disfunctional and setting you up for a bad dating life later on. Oddly enough, this could end up getting you asked out, because guys like "hard to get" or whatever. Just don't jump at the first oppertunity just because you want a boyfriend. You should want to date THAT person, not just have them be a filler. Be picky. ;)

Slap them across the face. That'll get their attention for sure.

In my experience most guys don't like girls who are all over them. The best way for me to get a guys attention is act like I could care less about him... To walk by and not even aknowledge him.

Flash them.