core strength is definitely key, the deep musculature in your upper body is what stabilizes the rest of your body and will help prevent from straining and injure other muscles/body parts. of course it's important to have strong legs - lift weights and vary your exercising. and this can't be stressed enough - stretch!!
here's a link with tons of good exercises, it's in norwegian though, but i'll put a link in here too with translation from norwegian to english (just make sure you click "fra norsk"(from norwegian)). there's a video enclosed with every exercise that shows how to do them, so you don't have to read the instruction. on the left side are the links to the different genres of exercises ("stabiliering/styrke", "koordinasjon" etc)