Douglas Labs Melatonin 3mg sublingual (I specify the brand and dose because every other type of melatonin I've tried doesn't even make a dent, while this one knocks me out like a hammer to the head. Before I tried it, I thought melatonin just didn't work for me. They should be paying me for the number of times I recommend this, but it's been a super help to me).
I also take lemon balm and hops, which help a tiny bit but not nearly like that particular kind of melatonin.
Staying away from any kind of electronics for an hour before bed has helped a lot as well. I didn't expect it to make nearly as much of a difference as it did, and wish I'd started doing that sooner.
Much of the time my sleep is much, much better now, and I'm eternally grateful for that. I do notice that I still have super crappy sleep and wake up in the wee hours of the morning if I've overdone it during the day. I think crappier sleep is just a regular part of my PEM, and one that shows up even when I haven't overdone it badly enough to trigger the worst of my PEM symptoms. If I'm careful about my energy expenditures during the day, I sleep a lot better.
Edited to add: Just to be clear, by "better" I mean that I can sleep through now and don't feel quite so dead in the morning if I get at least 9 or 10 hours. I still don't feel refreshed at all. Haven't yet found a supplement or medication that's made much of a dent in that.