There's no true formal prep-schools for this. Best way to be competitive to get these jobs is:
1. Have mountain flying/low level flying experience. Most ways to do this is through military tactical flying and/or Agricultural flying.
2. Have an A&P. Since most aircraft are deployed from their private contractor homebases and operated on minimum crew, pilots are expected to be able to perform at least their own minor maintenence. Spares are carried onboard the aircraft and "forward deployed" to the operating location.
3. For air attack and leadplane jobs, having training as a military Forward Air Controller really helps, since the job is practically the same, just with different results. Though not required, it does make one very competitive.
4. Any experience with wildland firefighting, ie-engine crew/hand crew/hotshot, is a must. Knowing the job that's going on on the ground, makes you understand that much more your support role from the air, not to mention strategy, how/why forest fire works....knowing the enemy and how to fight him, if you will.
Keep in mind, the aerial fire fighting community is small and close-knit. Most people that do it have been doing it for a long while and openings usually come from retirements, turnover, or death. Be prepared to be gone from home for long amounts of time deployed all over the western US (wherever you contract has you), making beans for money (as compared to the risk), and flying a dangerous mission in old aircraft. Most forest firefighting aircraft are old, worn-out ex-military and airline aircraft; there being a reason the military retired them in the first place. They are run hard, most performing a mission they were never designed for. An example of the risks are the recent losses of the C-130 in California and the PB4Y-2 in Colorado, both of which shed their wings inflight during swath runs.