I woke up the day after my first amphetamine experience feeling really sick. I could not even drink water. Whenever I tried it resulted in me puking. After a day and a half I started to shake uncontrollably and that really scared me. I phoned the hospital and got some good advice on what I should do.
After three and a half days I finally managed to eat some toast without throwing up.
Here is what I learned from the whole experience.

(NB: This info can of course be useful no matter what drug you've taken)

If you can not eat or drink anything and are feeling really nauseous, you should try and rinse out your stomach completely. That is the best way to speed up the recovery. If you don't do it, you will probably be sick for a much longer time.

Start by drinking a big glass of water. Wait for a couple of minutes. If you feel fit enough then try to shake your belly a bit so the water rinses your stomach more effectively. Then put some fingers, or a toothbrush, down your throat and empty your stomach completely.
Repeat this at least another couple of times. The idea is to get rid of all the shit that your stomach refuse to release into the small intestine. As long as it stays in your stomach, so does the nausea.

After you're through with the rather disgusting process above, it may still be impossible to even drink water without puking. The thing to do then is to mix one liter of water (about 0.23 gallons or 1.8 pints) together with one teaspoon of salt and six to eight tablespoons of sugar. This mix will be accepted by your stomach, even when nothing else is. Start out slowly though with only small sips.
Throw a little bit of lemon in to the mix if you find the taste too awful.

Now it is only a matter of time before you can eat. But don't rush it or you may have to puke again.

When you feel ready for some food I recommend that you start with something light, like toasted bread with marmalade, which is really easy on your stomach. And avoid meat, snacks and milk for another 2-4 days.

Good luck!