Some good ideas already posted here ...
If you are under stress and also have trouble with your mind continuing to run and keeping you awake, Skullcap (or sometimes spelled Scullcap) works well for a large percentage of people. Take 2 before sleeping, and if you wake up during the night, you can take another. It won't leave you drowsy, and is a good soothing tonic herb for the nerves also. Can be taken in the daytime as well. (Only thing is, if it DOES work for you, about 50% of people will lose effectiveness if they use it every night. If you alternate with something else or don't use it every night, you shouldn't have that problem.)
Passionflower, hops, and chamomile are all relaxing herbs.  You may be able to find them in a combination, or perhaps in a combination sleep aid tea if you like herbal teas. If you buy the tea, follow the instructions for steeping (and consider honey to sweeten if you want a sweetener). 
Valerian is helpful for some people. It is more like an actual sedative. It also helps people who have muscle pains to sleep. It is not habit-forming, but really shouldn't be used long-term either. (Don't buy plain valerian tea unless you have a strong stomach -- it STINKS!)
Melatonin is helpful for those people whose bodies don't produce sufficient melatonin naturally. Either it will work for you, or it won't ... if you already produce enough it won't make any difference. Often having a day/night disturbance (like changing working shifts, a very long-distance move, having the wrong amount of daylight, etc.) will be a factor for those people, but not always. This is another one you shouldn't really take long-term, as it can interfere with your body's ability to make melatonin and you will, in effect, need it after that. 
Lavender is a great soothing and relaxing scent. You can add the essential oil to a warm bath and really relax. I often put a few drops of it on the "body pillow" that I wrap my arms and legs around when I sleep. It can stain fabrics, but I use just one pillowcase for that one and I don't mind. Be sure to buy it from a reliable source.  There is more "lavender" being sold on the market than is actually harvested. Fake stuff won't work. 
Ummmm .... those are the main points I can remember right now, and what help about 99% of people. You can sometimes even find a product that includes quite a few of them, kind of a "shotgun" approach, if ever you are desperate and just NEED sleep. However, if you take that, you won't know which one helped you and if you want to switch to a single or more simple formula, you'll have to start over to find what helps. 
Oh, yeah, and I should say "I'm not a doctor, and none of this information is intended to treat, diagnose, etc. any medical conditon" etc. .... I didn't exactly word everything the way I would with customers, but that is the basic info, if it helps anyone. 
Sweet dreams!
trish