Magnesium is a great supplement in general. You can definitely have too much in your system and that is undesirable, but a bit before bed time along with 1-3mcg of melatonin work well for me. It is nice after workouts. I use magnesium glycinate in powder form, which is more bioavailable than some forms as well.

I also find sauna before bed is good. I have a bed chiller so I can crank up the sauna before bed and not sweat a lot. Generally if I sauna and take the aforementioned supplements I sleep well. Exercise also seems to help me out a lot. If I exercise during the day, and a 4-5 times that week in general, I tend to sleep well.