I always have trouble sleeping, whether its good MDMA, cut to death MDMA, and quite often when I'm sober.
I would like to say please DO NOT mix any of the drugs listed below! Except maybe weed. This is a list of favorite to least favorite. If I don't have my favorite, I take the next one down the list and so on.
Opiates are by far my favorite thing for coming down. Hydrocodone, codeine, poppy tea, or oxy (even though its mildly stimulating in itself.) They do a great job smoothing out the edge and make me comfortable (duh) to the point that the comedown is not entirely unenjoyable. Obviously don't take enough to nod out, thats asking for trouble. If you have no opiate tolerance (like me) then the equivalent dose of 10-20mg hydrocodone is a good place to start.
2nd is probably weed - no explanation needed. This doesn't usually work for me, but I would certainly try it before going on to number 3.
3rd best are benzos - Which I do not enjoy, recreationally or otherwise. However, they do smooth out the edges but without the comfort that opiates provide. With zero tolerance, I find the equivalent dose of 1-2mg Xanax good.
4th is ambien - I used to enjoy daily until I decided to quit. One is enough. Its a really strange mix, ambien and MDMA. Weird indeed. And it will help promote sleep.
5th would probably be GHB - very similar to the effects of alcohol (also a gabaergic) but not as bad for you and not a lot of liquid to swallow - as is the case when drinking alcohol. 1-2ml is a good starting dose for me. Be careful not to take too much. Drinking too much GHB is more forgiving than drinking too much alcohol, but that doesn't make it fun. Another plus of GHB is that it is not hepatotoxic (bad for your liver) while alcohol is, and the is no hangover, as there also is with alcohol.
5th is alcohol - I try not to drink away comedowns, and haven't had to in quite a long time. It's not at all healthy to mix with MDMA, it does not promote good (or even halfway decent) sleep and it leave me feeling worse the next day that I would otherwise. At most only a few drinks, the goal isn't to get staggering drunk, just get to sleep.
I find the supplements, while they sound nice and healthy, do not help me sleep. I always take 5-HTP, vit c and some B supplements after rolling. If I believe it works, maybe it does. Have I felt it working? No. But I still take it, because its easy and it might work.
Smething that does help is to force down a little food (bananas or oatmeal are good for this) and drink a couple bottles of water or glasses of OJ. You might not feel immediately better, but your body appreciates it none the less and will reward you the next morning by feeling less shitty than it otherwise would.
This is all just my experience, so don't give me shit if your opinions differ. Many people swear by valerian or other roots/barks/leaves/whatever. That's fine and more power to you. To each his (or her) own.