You need to wait for at least a week after you can't see any more signs of the ich. No more spots on any fish.
Draining is not really necessary. Siphoning the bottom often during extra water changes will help remove the cysts before they hatch into tons more ich looking for hosts.
Follow the directions on whatever you are using for meds.. and if it is copper based, you may not be able to put the snail back in. They're very sensitive to copper in their water.
Ich can usually be handled with raising the temperature to over 80 F, and sometimes adding some salt can speed up the death of the ich itself. Ich is more sensitive to salt than the fish are usually, so it can help.
High temperatures mean the water can't hold as much oxygen, so an air stone will help increase gas exchange, or let the water from the filter fall into the water, so it splashes down. However if you are using meds, you can't do the high temperatures too, as the combination can make the oxygen so low the fish suffocate. So if you've used meds, stick with them and follow the directions carefully.
I"ll go find a really great article on ick I have bookmarked. Explains the ich and treatment better than I ever could. Here is it.. have a read, it's really good.
http://www.cichlid-forum.com/articles/ich.php
