Consistency is really important to getting back into a normal sleep pattern. Everybody is a little different, so you have to find out what works for you. In general, though, you want to start by going to bed and getting up at the same times every day. It's also important to avoid things that disrupt normal sleep patterns -- for example, avoid caffeine (I generally don't drink it less than 8 hrs before I go to bed), exercise, or anything else that would get you wound up in the hours before you go to bed. I have also heard that exposure to blue light reduces the production of melatonin, making it harder to fall asleep, so that would be another thing to avoid shortly before going to bed. Generally speaking, any kind of stimuli that gets your brain or body going is something to avoid or cut down on in the hour or hours leading up to bed time, and finding things that help you relax before it’s bedtime is a good idea.
Alcohol may help you fall asleep, but the quality of your rest will generally be lower than without it. Even if you can't fall asleep right away, try to stay in bed with your eyes closed, with little or no outside stimuli. If your mind is too overactive, try clearing your thoughts -- essentially, any kind of meditative or breathing technique to help you relax is a good idea.
Most people are attuned to sleeping at night, so it’s generally a good idea to keep your room as dark as possible; cover up any LEDs or other lights if it helps. And, of course, minimize or eliminate any sounds that would distract you. Some people find that white noise or music helps them fall asleep; I don’t, I prefer quiet.
I also try to avoid drinking anything an hour or two before bed; if I don’t go to the bathroom right before going to sleep, sometimes the need to go will either wake me up during the night, or it won’t, and I’ll sleep more uncomfortably during the night.
People with genuine insomnia would probably have more to add, or have more unique methods of dealing with lack of sleep. A friend of mine is an insomniac, and he occasionally has to push himself through several days of no sleep before his body finally shuts down; then he may sleep the sleep of the dead for 12-16 hours.
EDIT – sleep aids may help in the short-term as well, but I don’t recommend using them a lot, to avoid from becoming too dependent on them. A single Tylenol PM (which has 25mg of diphenhydramine) once in a while is usually enough to help me fall asleep if I need it; 2 makes me groggy for a bit the next morning. (I don’t take a lot of medication.)
That’s pretty much everything I can think of at the moment.
