I stay at St. John every year and I almost could write a travel guide on it. Here is some of the more important information.
1)The first thing I would recommend is to take a day trip to st. john. the beaches there are a lot less crowded and practically the whole island is national park.
2)Have a passport. You don't want to miss out on what the BVI has to offer being so close to it.
3)If you want to spring for it, charter a private boat trip. They will take you some truly awesome places throughout the virgin islands. I recommend this guy
Virgin Islands This Week: Nate's Custom Charter's. He is incredibly nice and will do the best to take care of you.
4)If you do take the trip, make sure to ask him to take you to the Indians, Willy T's, and sandy spit(Tiny island straight out of a corona commercial). Also, soggy dollar bar is really cool. Its the place that came up with the pain killer and once you go there you will understand how it got its name.
5) Dive the wreck of the rhone. I havent done it yet, but it is supposed to be an amazing wreck dive.
6) Try to avoid the big touristy areas, especially when the cruise ships let off. Downtown st. thomas gets mobbed!!
7) Im not sure how much this applies to st. thomas, but in st. john its a must to rent a car. I'm assuming its the same way there. Its really the best way to get to all the nicest beaches.
6) Bring back rum. You really can't beat $5 a bottle