My experience:
1. Find out what's important to HER. For instance, birthdays never seemed a big deal to me, but she likes a little bit of a celebration -- nothing fancy, mind, but a few ribbons here & there.
2. Listen beyond the words. Something that doesn't usually bother her might get to her sometimes; find out why she was unhappy to start with. Work, relationship, family, ...
3. Do something unexpected and nice once in a while.
4. Trust her. I know it sounds obvious, but I was hesitant to tell her about some things first. I did, anyway, and eventually found that it's much less a big deal when you're in it together.
Good luck!