ZMA and Meletonin work good.  I always keep both of them with me.
ZMA = Gives me a very deep sleep.  I wake up feeling like a million bucks.  Also really vivid dreams.
Meletonin = Doesn't really change sleep quality, but helps you to fall asleep.
As far as tolerance, ZMA seems to give the same results no matter how long i use it.  But with the meletonin I dont take it more than 2nights in a row or else it gets very hard to sleep the next night with out it.
You can buy either of them pretty cheap online.