The best way to celebrate the birthday of the woman in your life is to know what she likes and provide it that day. However, to make the day stand out, do a build-up where you give her some kind of gift in each of the days before her birthday.
A week before her actual birthday, surprise her with a small, inexpensive gift. Make it something so small she may not even think of it as a gift. For example, buy her favorite cupcake and place it on a desk or table where she isn't expecting to find something she likes.
For a second gift, do something you usually don't do for her. Clean up something without her having to ask you to do it, or make something in the kitchen from scratch. If you're not good in the kitchen, this may fall under the "it's the thought that counts" category. Regardless, she should enjoy the effort.
Create a one-of-a-kind gift like a collage of photos, or a "mix-tape" CD. Place this in a location she isn't expecting to find a gift with a note.
For the next gift, give her something you know she likes to wear, be it clothes or jewelry. This gift shouldn't be that expensive, but something she can show off to others a few days ahead of her birthday. By now she should be on to you, but that's fine.
On her birthday, give her the "big" gift, whatever that may be. The final gift just needs to be the culmination of what can be a week-long celebration of her birthday.
If she does pick up on your gift strategy early, hide the gifts around the house. Write clues on sticky notes, and let her find them. She may enjoy the search alone, and it could make the actual gift even sweeter to find.