Because you like this girl, right?
1. Never stop flirting with her.
No matter how long you have been dating, friends, or even married, you cant stop. Why? Because that's a part of why she likes you. You tried to get her, and now you have her. Keep her, by flirting with her even when you don't have to.
2. Oh, she doesn't want gifts?
Too bad. Yes she does, even if she doesn't know it. The trick here is matching "No price is too high" with "I'm not trying to buy you". How do you do this? Happy Birthday! How much did this cost?" STOP Don't tell her. It doesn't matter. You didn't buy it for the price, you bought it to make her happy. If she persists, then everything you have EVER bought is now $10. How much was this paper crane you made? $10. This movie you bought for us? $10. That Gold and Diamond laced dress? Obviously, it was $10. To put it short, even if she said she doesn't want a gift, you get her one. or you make one.
I understand, it's not that you don't care. You just cant always think of her schedule. It's tough to remember every little thing she is up to. Solution, when you DO remember, go over the top with little details. Easiest way to do this? Get cards. Woman, for some reason, like to get hallmark cards. For extra credit, get one with nothing in it, and write your own card. For events that aren't card occasion, simply insist you go. clear some time, go to her [Hobby] recital and tell her how good she did.
4. Make her feel sexy
This one should be easy. Compliments, be erotic, make sure she knows you want her. WARNING, make scare off abstinent women, for obvious reasons. Regardless, makes sure she knows you like all of her.
5. Find something to do at home
Find hobbies. A relationship will only go so far with physicality. And dates get repetitive. Find something you can do together at home besides movies and making out. Games, art, music, anything. Find it, suggest it, test it, enjoy it.
5. Don't let her friends/family hate you
You talk with her alot right? You talk about everything right? Not about you. And trust me, she will find someone to talk about you to.
6. Get emotional
Eventually, any girl will want you to open up to them. This one killed my last relationship. Personally, I can't stand putting my real problems on other people. But keeping an emotional distance while in a relationship measured in years will make it crumble. By this time, the girl wants to know all of you. You have to let them in. Jumping in to a relationship all emotional is a good way to scare most women off, but showing emotion late in a relationship is the best way to keep them around.
All in all, relationships take work. As long as you stay attentive, and keep caring about her, you can keep things interesting and alive.