I normally just do hamstring, quadriceps, calves, groin stretches. And then some arm stretches just because it's a habit from all sports. Usually I forget about stretching until I'm at the hill so that's all I do. If I completely forget until my boots are on, I'll at least do hamstring and quad stretches.
Sucks about your back. Try doing some back stretches as well as strength training exercises that strengthen your back. If the pain is from an injury that you remember, rest it and see a doctor if needed.