Oh, to be young and in my college days... never ever again!
Like you, I thought I had to do or say some kind of tricky stuff to get girls to give me their numbers. Turns out, guy, it's not really that hard.
You're in college, so there's always plenty of time between classes to run into girls and start small conversations. So, let's say you've just finished a class, and you know one of the girls in your class is going in a similar direction as you to another class. You catch up with her en route and start a random conversation about something class-related:
YOU: Hey, did you know what the teacher was talking about when they said _____?
If she responds, she's at least open to conversation. It doesn't have to be some kind of fancy opener - in fact, the more basic, the better. Start walking and talking, and adding on to the conversation or changing the subject to something else. Then, just as you two are at the point where you're about to go in different directions:
YOU: Alright, well, I gotta head this way to class. Oh hey, real quick, let me get your phone number.
That's really all it takes. REALLY. You don't ask them, you just say it as part of the conversation. Amazingly, doing it this way will get you more numbers than if you're acting all nervous or trying to ASK them for it. It's almost as if you're subconsciously telling them "I know you want to give me your number, so go ahead and put it in my phone." If she decides not to give it to you, don't treat it like a big deal - for example:
HER: Um... you can look me up on facebook
or:
HER: Um... I can't, I have a boyfriend...
or:
HER: Um... well, I mean, I'll see you in class tomorrow, right?
Just laugh it off like it's no big deal - "ah, okay, cool, see ya later" - and walk off.
But yeah, really simple stuff, man. And hey - if you need more tips, click on the link in my signature to hear my daily podcasts!