I'll agree with the other posters. Melatonin works very well to help you unwind. Blackout shades/curtains. Some kind of white noise. I've had colleagues who purchased a portable UV lamp to keep at their seat at the nurses station. That way, their body got the 'sunlight' it needs, then you take melatonin before bed, simulating that regular cycle. Have a small snack before you crash, that way you won't be waking up at lunchtime because you are starving. Don't come home and turn on the tv or computer, maybe read a bit, but no electronics or stuff that could stimulate you. And try not to switch your schedule around too much on your days off, it will just mess with you too bad. I know it is tempting, because you will want to be able to hang out with friends and family. What you can do is something like this:
On work days, shift from 7pm to 7am. You usually get up at say...4pm to get ready and get to work on time. After work, get home and in bed by 9am-ish.
On first day off, get home from work, go to bed at 9-ish. Get up around 1-2pm. You got 5 hours or so of sleep. Up until, say, 3-4am.
If you can keep that kind of schedule on your days off, you won't mess up your sleep/wake cycle too badly.