Have you looked at some natural ways to help get to sleep earlier? No TV in the bedroom, same time/routine for going to sleep each night (getting on a schedule), an hour or so before to wind down, turning the cell phone off?
Not to say any of the supplement advice isn't good or necessary, but if I had the issue I'd research and start with the non try taking this answers if it was me.
Also I realize that depending on what you do the schedule thing may or may not be possible.
This right here. Lifestyle factors ought to be optimised (if they aren't already) first. What's your schedule like? Do you have a regular bed-time and a regular wake time?
I can speak from years of experience with sleep issues that were down to a variety of factors. Tried a number of supps, most of which have been mentioned here (except melatonin), and they helped at times but I've found more than anything it's the lifestyle factors that are most important.
Regular time to bed is critical, but so is an appropriate length of time to wind down. The worst thing you can do is try to make large changes in habitual time to bed. For instance if you're not getting to bed until 1am, trying to sleep at 10pm is likely to be entirely counterproductive - make the changes gradual, say 15 minutes at a time. Removing excitatory stimulus like tv and computer from the sleep window will help an immeasurable amount, but as Geoforce said it may not be possible. In that case there's a few things you can try.
If working on your computer late at night is simply unavoidable, there's a lightweight desktop app you can download for either PC or Mac called f.lux. It tints your screen red (you can customise colour temperature) automatically as the sun goes down. The idea is that the photosensitive ganglion cells which ultimately play a role in pineal gland regulation of melatonin production respond most strongly to light in the blue-violet spectrum. If you filter that out you reduce it's impact on natural melatonin suppression. Having used it for some time now, I can't say for sure what, if any effect is has had, but I do find it extremely irritating if I disable it for any reason working late. There is plenty of supporting research on their site (though some of it inapplicable to computer screens - I had believed that a screen couldn't generate sufficient luminance which is supported here: ww w.ncbi.nlm.nih.gov/pubmed/21552190, but it is worth trying.
Guided or transcendental meditation techniques are also well worth practicing too. Can work wonders for sleep induction, and plenty of information around the net.
The most effective thing for me though has been listening to music. Anything that relaxes you and reduces mental chatter will do the trick, but for me that's downtempo idm, ambient soundscapes, drone, and sometimes field recordings. There are stacks of netlabels that produce creative commons albums that are free to download in these genres (let me know if you want links) and I've accumulated well over 100 over the years. Some of the labels are better than others but it's all a matter of personal preference really. Either way, while my sleep issues have largely been controlled for years I do still run into nights where I can't sleep. First thing I always do is put the headphones on and listen to an album and 9 times out of 10 it does the trick.