Well, besides just going out and surf alot.....
I personally believe the best training for a sport is the sports it self. There is nothing really that compares or feels the same as surfing. The bottom line is that the more you surf the better you get at it. But If you cant get to the ocean here are my recommendations for preparation for surfing.
Yoga is less stress on the joints and actually works on some of the similar muscle structures that is useful for surfing. Core strength is very important to surfing because you are constantly pulling and turning your mid section. Your power and speed comes from the stomach muscles and yoga is especially good to target those muscle groups. I would recommend to get yoga for surfers the video. You can cell some clips on you tube or go to get the CD or books.
Indo boards are fun and can be quite a good surf trainer. You can pick one for really cheep and it will help your balance skills!
Here is a video: