Master the thoughtful gesture.
Being romantic is often synonymous with corny, cliched moves you ripped off from a rom-com. Your girl doesn't want rehashed romance, nor does she need some elaborate scheme. She only wants you to be thoughtful.
Toss the red roses, and opt for a subtle gesture that says, "I know you, and I thought of you." Bring home her favorite candy, just because. Unexpectedly make reservations at that restaurant she mentioned a few times. If you live together, do the dishes without being asked.
Contrary to popular belief, most women are not high maintenance. They want small, thoughtful gestures that show you're thinking of them and you care. That's enough to satisfy even the most demanding of women.