I struggle falling asleep sometimes as well. Actually, a lot of the time. Here's what helps for me:
1. Don't bring my laptop into the bedroom. If it's around, I keep giving in to the urge to check HN or Facebook or Twitter or my email "one last time" every 15 minutes or so.
2. I drink a glass of milk and take a melatonin tablet about 30 minutes before bed. Melatonin helps induce drowsiness.
3. Turn the light off. Seems totally obvious, but if I'm reading in bed, I may have my lamp on. Sometimes I just have to force myself to turn the f!#kng light off so I can fall asleep.
4. Turn the AC down. I sleep better when it's a little chilly, so I turn my thermostat down a few degrees a little while before bedtime.
5. Daydream. I just start running through some kind of scenario in my head and let the fantasy run until I fall asleep.