Don't over-do the romantic stuff, so that when you are romantic, it's special.
Don't agree with her on everything she says for fear of offending her.
Similairly, don't claim to like everything she likes. Music, etc. Obviously, if you both happen to have the exact same interests (unlikely) then you can say so, but girls don't want someone who'll just agree with them all the time (the ones who do are idiots, we ignore them).
Impose equality in the relationship. My girlfriend and I detest all that "ladies first" crap. If I cook, she washes up, and vice versa.
Don't act extremely differently around her than you do when your friends are there. It puts girls off when they find that the ultra-pleasant guy they're interested in is a total spastic when he's with his friends. It'll make her think you're being fake, so basically just be yourself.