Ride moto for long periods of time. If you not into that try going for a xc ride on your dh bike. When your tired try to pin any dh sections I find it helps. Dirt jumping helps to it gets you comfortable with moving the bike around in the air.
ride all the time, hike back up trails sometimes instead of shuttling, ride a hardtail every once in a while, there are a lot of things you can do. also ive found that riding bmx helps me ride much smoother and helps with turning. but the best way to train for downhill is just to go out and ride as much as you can.