Make sure that she's single ahead of time. Girls HATE being single on Valentine's day more than any time of the year.
Step2
Girls want to have someone special for Valentine's day. Being single on Valentine's day is LONELY. Ask her out about a week before the big day if you have time.
Step3
Make the date casual, but be firm. Say "Do you want to get a slice of cheesecake with me on Valentine's Day?" instead of "Do you want to do something sometime?"
Step4
Give her one flower, not a red rose, that's lame and predictable. Put a little thought into it. Don't buy her anything expensive. One flower and a slice of cheesecake/cup of coffee will do.
Tips & Warnings
She will be so happy not to be single on V-day she will say yes. Use the date to show her how great you are and score a second date!