Apart from getting your tickets cheap.....
the main tip I have is to PLAN your time and visit there at the theme park.
I have only been to Disneyland, but buying and reading an "Unofficial Guide to Disneyland" was one of the best things I have every done in terms of cost to benefit ratio.
It was chock full of great advice and showed how to get more rides in per day...minimise time in queues etc...and generaly enjoy the experience more.
If you have kids under very young (ie ride -wise), "the baby shuffle" was an amazing tip that allowed us adults to get rides in more quickly by otherwise halving the queuing time that we would have had to endure. without this tip we probably would not have smaple anywhere near the quantity of adult rides that we did.