melatonin works very well for me, though I've read some conflicting reports on its safety. I just take it when I need it or when I'm traveling.

Valerian does work well, but sometimes I find there's a bit of a "hangover" the next morning.

I must be weird, because GABA actually makes me agitated. My face would get all tingly, and I get a weird sensation/taste in my mouth. I once took it to sleep and as I drifted off suddenly bolted upright with my heart racing and feeling rather frightenend. I didn't put two and two together until I took GABA again a few nights later with the same result. I guess we're all wired differently!

5-HTP's supposed to be good for insomnia, too.