I've been in a long-distance relationship for over a year, and it's going well. Of course, some things that work in my situation may not be perfect for yours (and if they do, you may have to make some changes), but I have done a couple things she really loved.
A few weeks ago I paid her mom for two dozen roses: I live 4 hours away, so I often have her set things up for me. I sent a handwritten letter, which her mom picked up when she went to get the roses. While my girlfriend was at work, her mom scattered a dozen roses around her room and put the other dozen in a vase with my letter tied to it. She loved that.
Also, know her interests. Knowing that my girlfriend likes video games, I coded a small visual novel for her on Valentine's Day: an interactive love letter, if you will. That worked well.
I would also recommend looking up info on the 5 Love Languages, or just get the book by that name. Amazingly helpful information on what kinds of people respond to what kinds of affection. Highly recommended.