I'm so absorbed in the business aspect of my practice that I've given myself some seriously ironic insomnia. At some point my subconscious convinced itself that obsessing over meaningless details at 3 a.m. is the key to some kind of marketing revelation. If you've ever done this, I'm sure you already know that it is actually the key to a really crappy next morning. So what do we do?
The first step is to honestly make sleep a priority. It's easy to think that whatever is keeping you up at night (work, school, a relationship) is more important than sleep, and for a few days that might be true. Over time, though, being constantly sleep-deprived will ruin your career / grades / relationship anyway, if it doesn't kill you first. Here's how to make sleep a priority in 20 minutes per day.
The first step is preparation: make your bedroom a sleeping room. Move the TV, the laptop, the iPad, the box of your kid's toys, whatever. Anything with a screen, especially, should be kicked out. If you live in a studio or have one room in a shared house, find some way to convert your room at night, so that distractions are literally covered. When I lived in a studio, I put a blanket over my computer desk at night. It sounds silly, but it did help (plus it covered the flashing laptop battery light).
20 minutes before you want to go to sleep, get a notebook and make a to-do list for the next day. It shouldn't take more than 10 minutes. If it takes 20 minutes to write your to-do list, you might have unrealistic expectations about how much you can get done in a day. Once you write your to-do list (which is probably at least partially related to the thing you've been obsessing about), forget about it. It's on paper, you don't need to remember it. If the thing you're obsessing over is emotional, write that down instead of a to-do list. Write for 10 minutes only about whatever is bothering you, then actively let it go. You should do the writing in a room that is not your bedroom.
When you're done writing, you can get ready to sleep. Don't check your email one last time, don't turn on the TV. Sit on the edge of your bed (or on a chair in your bedroom) and breathe. In the beginning, this will be 10 minutes of obsessing over how you're not focused enough on your breath, and that's fine. It doesn't get easier, but it does get more effective if you keep doing it for a few weeks. Don't worry about whether you're doing it right, just sit and breathe. Set a timer (not your alarm clock) for 10 minutes so you don't have to watch the clock. In the beginning, you'll watch the clock anyway, and that's fine too.
Relaxation is an active process. We live in a culture that encourages us to collapse, rather than relax, because if you still have energy you should be using it to be productive. Use that extra energy to practice letting go. It's harder than being productive, but if you stick with it, it's worth it.