well ive done crystal three times and all three i took it orally, but i do have experience with other amphetamines like ritalin, adderal IR and XR, vyvanse, and various others.
to answer your question the best way ive read is benzos and sleeping pills.
when i come down ill usually take melatonin if ive dosed early enough in the day when im coming down around 10 or 11. if not i will take clonodine or dramamine. but if i take those to early they dont work unless i take a lot.
a lot of people reccomend benzos or weed. but try some different ones out to see what works best. ive slept six hours after taking amp so every person is different
edit: also the best way to get rid of a bunch of the comedown symptoms is to eat two hours before (if you pop it), a small protein snack a few hours after you take it using your ROA, and a meal when your coming down. and of course hydrate throughout your whole high.