Keeping the relationship strong through the summer?

I know there are already similar questions, but mine is a bit different. Here's a brief summary:

I started to date this girl that I have known for ever, but I wasn't attracted to her until last year. She broke up with her bf a couple of months ago. I gave her sometime and then made my move. We went on a double date which somewhat flawed because my friends date didn't show up.... Regardless, her and I still had fun. About 2 weeks ago I invited her to my house to go to the beach and use the pool and then get a bite to eat. She enjoyed it and so did I. She showed all of the signs of liking me, finding me interesting, and flirting. I'm apparently the first one to ask her out and it is my first serious relationship.

Now the question:

I'm leaving for the summer this year and may not return next year (won't know till the end of the summer). I'm taking it slowly with her because of this. Now if I do come back, is there anyway to keep her "wanting me?" I have her number and I'm friends with her on fb. I'm leaving in 3 months and I see her on Mondays every week for class. I also invited her to my bday party next week.

Main thing I'm worried about is that I will kinda fade out of her mind because I won't physically be there and we will lose this closely bonded relationship that I want to evolve to into something a bit more serious. She's my first love. She's got the looks, the personality, and the soul. She's simply beautiful in every way.

So how can I keep this going even though I wont physically be there?

Any suggestions would help.