There are a couple things to try to get your balance more centered on the board.1) Keep your front knee bent all the time while riding. You will notice your balance will improve greatly.2) Increase the forward lean on your front binding. This will assist in keeping your front knee bent more as well as improve turn initiation.
This is great advise, be aware for the first few times riding you might find your leg burns a bit but stick with it as this all helps the muscle memory