Practice by... practicing. Do some sample competitions (usually you can get older versions of the test) and see what kind of problems come up. Some you'll know how to do, some you won't. Pick out the ones you don't know how to do, learn how to do them, then find more problems of that type to practice on. It depends on what kind of competition it is, in high school a lot of them were big on geometry so learning obscure theorems involving circles and secants were worthwhile, depending on what kind of test you're taking different subjects will be important to know