I'm sure someone has anyway suggested melatonin? I usually take 1 or 2 gravol or benadryl to pass out after I'm done, but that's not natural I guess. Oh, and ear plugs, always. Hmmm... I guess you could try meditation? I'm not sure how successful you'll be with tricking your mind into sleeping after amphetamine. Honestly, if melatonin or other sleep aids aren't an option your best bet is probaby to just relax, put some ear plugs in, try and get comfy and let it run it's course. Depending on how long since I last dosed, I can't usually fully fall asleep but I can go in and out of sleep, teetering on the edge and usually feel refreshed after.
Sorry... this was probably totally unhelpful. Melatonin is pretty natural, we already produce it and it doesn't suppress your respiratory system at all.
Honestly, after I've just binged on speed for however long, it's all the same to me. Natural or not, the best thing I can do for myself is to get a real sleep.
On stimulants it's very difficult to release your grip on consciousness; the mind refuses to quieten and constantly analyses stuff in an infuriating way (like thinking about thinking about not being able to sleep because you're thinking too much).
I find a distraction which doesn't inspire thoughts is the best way to free your grip and sink into sleep. Max Richter has composed and recorded an eight hour piece called 'From Sleep' which lulls you into sleep and accompanies the different stages of R.E.M. and deep sleep. Apparently it replicates 'delta waves' which mimic the brain patterns found in deep sleep (I don't really understand the science behind this, but the music is relaxing in a repetitive way conducive to 'zoning out'). If you search for 'delta waves' in YouTube you'll find plenty of lengthy delta wave compositions which seem to either resemble monotonous Buddhist style gongs and chanting or new age type electronic repetitive trance which claims to incorporate delta waves in the noise.
Sometimes, largely by dint of the repetition and unobtrusive low octave sounds, this stuff occupies my thoughts and prevents me from analysing which helps me drift off to sleep.