Laying the groundwork happens on the first day of class. Scan the class for the flyest girl, and sit next to her; not on top of her (let a girl live!), but close enough to make comfortable conversation. 

This works for several reasons: For one, she'll be aware you exist. Which, in a lecture hall with 100 sardined students, is a solid start. Your proximity will also allow for natural conversation (shouting across the room doesn't quite work the same way) on a daily basis. 

Bonus: When the professor asks the class to pair up with a partner, she'll automatically look to her left or right. One-on-one time, effortlessly secured!