100% approved by our army of gals and our field report earns us a few bucks.
Get her a nice vase for that bouquet. Flowers are nice for a while, but they don't last forever. A beautiful vase though (not some cheapo) is something she'll hang onto for the next time you buy her flowers. It will also turn her bouquet into a complete gift, rather than some loose bunch of stems and petals.
Surprise her once in a while. Gals always like getting flowers, but they especially like getting them when you're not celebrating any particular occasion. If you want to make her melt, pick a day at random and give her some flowers for no reason at all.
A giant bouquet isn't necessary. Arm yourself with some basic information by finding out her favorite types and colors of flowers. Then get her something to match. (Mix it up from time to time though.) It's okay to go for a small or inexpensive bouquet if you think that will fit her best.