The main reasons people have for not falling asleep easily is stress, but stress comes in many forms.
Too much coffee, eating too close to bedtime, and exercising too close to bed all increase your metabolism and consequently make you feel more awake. Also playing or watching anything frenetic just before bed (i.e. playing FPS, racers, watching big action films) all cause arousal (not the sexual kind) and you need time for that to calm down. So if you don't give yourself some downtime you struggle to relax in bed. If you're constantly doing stuff from dawn til dusk and never give yourself a chance to just sit and relax without distraction (i.e. no tv, no games, no books, nothing) you're kind of not give your mind a chance to absorb everything. You go to bed wired and you're head just want stop thinking
Overheating. Your body cools down ever so slightly when you go to sleep. If you make yourself too warm in bed it could be keeping your body from resting phase 2 sleep (relaxed but barely aware your still awake).
Being too hungry or too thirsty can also keep people awake. You'd be surpised how many people get used to ignoring their stomach they don't realise they're hungry or dehydrated. Students are particularly susceptible to this (especially when they're scrimping to save money). Make sure you've had a decent meal and drink, but at least two hours before you go to bed. People naturally feel sleepy around 2 hours after dinner (on average).
Anxiety/Mania/Mind-racing. Linked to the first one but those causes are more immediate and environmental. Here I'm talking about life stressors. If you've got stuff to deal with, good or bad you might be keeping yourself awake trying to figure it all out. Whether you're conscious of it or not. The best cure is to figure out the problem of course but failing that you need relaxation exercises. The simplest and most effective I've seen are;
1. 5x5x5 breathing. Breath in for a count of 5, hold your breath for a count of 5, breath out for a count of 5. Slow, not too forced, don't burst your lungs. Just long, slow, big, deep breaths. Repeat about 4 or 5 times to lower your heart rate.
2. Mantras. Doesn't have to be fruity buddhist ones. Just a repeatable word or phrase that you mentally force yourself to repeat. Personally I used the word "Spoon" It's meaningless and very dull. The idea is to to focus on the key word and effectively stop thought intrusions. As soon as you notice your mind racing off on some tangent "SPOON... SPOON". Visualise it. This takes practice but it will train your mind into learning that it's now time for sleep.
Environmental factors - The room is too light, too dark, too stuffy, too cold, too airy, too noisy etc. Have a really good look at your bedroom and see if there's anything that might be putting you off sleep. Some people have realised having a window open makes them feel slightly vunerable and putting a secure lock on it helped. And as someone said making your bed JUST for sleep and nothing else is another good way to train you mind, effectively saying "now is the time for sleep".
And one last piece of advice - STOP CARING ABOUT IT. Nothing worse when you try everything and nothing works and you get frustrated. Just remember, so what if you suffer insomnia. Most people can barely stay awake, consider yourself lucky. There's no rule that says you have to sleep 8 hours a day, some people never need to. So why not treat yourself. If you can't sleep then get up and read that book. Do the washing up. Watch that cheesy movie. If you're relaxed then sleep isn't as essential as you think. If you spend 4 hours just chilling out and 4 hours asleep then you're probably getting a better night's rest than most people.
Good luck and do try some of these things. I can guarantee they are tried and tested methods I've seen work on most people and it many cases they seem to work miraculously. People always seem to vary parts to suit their needs but these methods seem to work and like most things the moment it works you suddenly don't need to use them anymore.
