I often have trouble, but it's more like I can't wind down - not tired, wide awake in fact, when I really should be sleeping.
What often causes this for me is playing videogames too late, especially games with lots of action, FPS shooters especially. I mean, I used to play a lot of Battlefield:MC, and trying to sleep after that was always a nightmare, it's like my mind is still playing the game subconciously. It's crazy, kinda like a waking dream, even dying and respawning - so it's not like my brain even let me be awesome. When your brain is working overtime, then you suddenly stop and try to sleep, sometimes it takes a while before your brain realises and settles down. Crazy I know, I hope some escapists have had weird videogame dreams like that before - my brother has, so I know it's not just me! - If you haven't then imagine getting a game stuck in your head, much like sometimes you might get a song stuck in your head. It's weird and annoying but kinda cool at the same time. Lucid videogaming if you like.
Anyway - Avoid high caffeine drinks, avoid playing videogames an hour before you go to bed, and have a snack, maybe some waffles or a bagel. Also, when you are suffering, get up and do something, anything really - try and knock your brain out of the gear it's stuck in - watch a little TV, read something, and wear something warmer as well for when you try and get to sleep again.