Here's a couple tips (no pun intended)

1. Trim your pubes. If Harold and Kumar taught us anything, it's that trimming your pubes "totally makes your johnson look bigger." (Also, girls don't want to see cousin it in your pants)
2. Just give it hell, man. There was a time when I didn't know what I was doing either. Maybe you can have the girl get on top?
3. WEAR A RUBBER, especially since you said she's had sex a lot.
4. Last resort - take her downtown.