Answer
First, and most important, a person suffering from insomnia should get to a doctor. There are many possible causes, and you want to rule out all the medical ones first. I am a clinical social worker and not a physician. Here are a few things that could interfere with sleep:
Apnea. Ask your sleeping partner if you're not sure. If you suffer from it, your partner will know. This is the disorder where you stop breathing for a few moments, and then snore loud enough to remove the wallpaper. Apnea can be related to weight, and it can also be "essential", or neurologically caused. There are treatments, including CPAP (Continuous Positive Air Pressure Pump).
Alcohol. Part of the misuse of alcohol can be the mistaken idea that it helps induce sleep. It doesn't, at least not in the long term. Anyone who has struggled with this addiction will tell you about the many destructive consequences of alcohol. Incidentally, if I wanted to create depression in people, I'd give them alcohol.
Mental health issues. Depression and other mental health conditions can cause or co-exist with insomnia. The different patterns of broken sleep can indicate the kind of issue being dealt with. For example, some people can fall asleep quickly, but wake very early and cannot return to sleep. Others remain awake for most of the night and might get short periods of fitful sleep at daybreak or during the day.
Irregular bed times, or working alternating shifts. Your body does get used to a rhythm (or tries to) and if you attempt quick or frequent changes, you will run into trouble.
A television in the bedroom. No, no, no. It doesn't help you sleep. The bedroom is for two of life's special activities--- and TV isn't one of them.
Bad mattress. You know if you have one. Make a change, especially if you're getting those strange aches you never had before.
Eating late. Food increases blood sugar, and blood sugar tells the brain "It's time to be awake!"
Caffeine late in the day, or throughout the day. Insist all you want that it doesn't bother you. Yes, yes it does. Even if you "sleep", caffeine will disrupt the natural sleep rhythms that help to make sleep restorative.
Bright light (PARTICULARLY) those full-spectrum lights, late in the day. These lights are wonderful, and when used properly they can have a very positive effect on mood, especially for people who suffer from Seasonal-Affective-Disorder (SAD). But they must be used with great care. Used late in the day, they have the effect of re-setting your circadian clock. This can be corrected, but why mess with it at all? Use these lights under the direction of a physician or knowledgeable professional.
Napping during the day. I know, you can't sleep at night, so you have to sleep some time, right? Try to work through it, and avoid napping.
Try these tips:
Get good physical activity during the day. At least, get some natural sunlight on your face early in the day. (Don't stare at the sun, of course, but get some sun on you.)
Have a regular bed time, and have a dark and reasonably cool bedroom. Make your bed; this does help to promote good sleep. Take a shower or warm soak before bed, but give your body about an hour to cool down before you get into bed. Cooling is a signal to the brain that "it's time to rest".
Generally, if you are having trouble falling asleep remaining in bed is not a good idea. Get up and do something quiet and relatively non-stimulating until you feel drowsy enough to hop into bed. Remaining in bed only serves to associate 'being in bed' with 'struggling for sleep'.