Tara,
I can relate. My big sis graduated this year and is moving in July to another city.

We are close and I am going to miss her very much! Granted, she will only be 4 hours away, but I will probably get to see her once every month or two.
Like XOAlumXO said, writing and phone calls are key. My big sis and I chat on AOL, even when we are in the same city, so I know we'll always have that form of communication. Also, thank God for nation-wide calling on cell phones.
Your big sis will not physically be by your side, but she will be forever be in your heart.