How do I keep a girl interested?

My problem with girls is once I have them interested I either come off too strongly (either by telling them how I feel or talking to them all the time) or I don't show enough interest and they move on. Lately I've been spending time with a girl who I feel is interested in me; we've gotten coffee, done homework together, gone to a comedy show and texted a lot. One night while walking her home I kissed her goodbye on the cheek and, while it wasn't romantic or anything, it wasn't super awkward either, and she talked to me the next day. Now winter break is coming up (we're in college) and I'm just worried that all this buildup will be lost over the break.