With this in mind, what I am trying to get at is: stress. The amount of stress you give your computer, the less willing it is to shutdown quickly - same goes with your body - the more worried and stressed you keep your mind going, the less likely you will fall asleep sooner. So stop thinking about the crap that happened earlier today, stop worrying about what's coming tomorrow. Taking some deep breaths helps to slow things down!
Internally, your body should be ready to drop down into hibernation. What this means is that you don't want your organs and what not to be working intensively. You should empty your bladder before going to sleep. Obviously, you wouldn't want to have your bladder waking you up in the middle of the night to interrupt your sleep! The same goes for your stomach. Ideally though, you should not go to bed on a completely empty stomach!