Keep communicating. Just because you can't see each other every day doesn't mean that you can't keep in touch. Email, phone calls and instant messaging are all great ways to keep close to one another. Make it a point to call or IM each other at least once a day. Talk about how your day has been, and don't forget to let the other person know that you miss being together.
Get together as often as possible. Just because you live hours apart doesn't mean that you can't schedule a few get-togethers during the long summer break. If you have the money, fly or drive to see one another at least once a month. If funds are tight, try to meet halfway through and spend a long weekend enjoying the sights at an amusement park or other tourist attraction.
Send care packages. When you want to maintain a relationship over the summer, you have to get creative to show the other person that you care. Send him cards and little care packages throughout the summer to let him know that you're thinking of him. If you see a trinket that your significant other would like, send it to him as a reminder of your devotion to one another.
Be honest. A relationship only works if both parties are completely honest with one another. If you're having trouble making it through the day without your girlfriend, let her know and try to arrange a meet-up to reconnect.