Check out some of the threads on the Long Distance forum.
Look at the training information on http://www.ultracycling.com/
Ride a whole lot. Do long back-to-back days. If you're planning on StP as a back-to-back century, then get to where you're comfortable with b2b 65-75 mile rides on a weekend. If you're planning it as a double, aim for having a double metric under your belt before StP, and not feeling like you're going to die afterwards.
I've talked to many people who've done StP and they all say it's a very easy route, and if you can ride a strong century then you can finish StP.