I've tried pretty much every Over-The-Counter sleep aid there is. Melatonin is indeed the most effective one I've tried, but my experience is that tolerance builds up rather quickly. I can use it every night for maybe 2-4 weeks, but after that, the amount required to achieve the same effects become absurdly high.

Also, one thing I like about Melatonin is the lucidity of the dreams I experience while sleeping with it. Colors, sensations, and my memory of the dream the next morning are very intense.

Currently I'm using Valerian root[0], which is all-natural, and I don't develop a noticeable tolerance.

0: http://en.wikipedia.org/wiki/Valerian_%28herb%29