What type of treatment are you using? Generally continuing treatment for 1-2 weeks more is best, that assures that the ich are finally dead as 1 week is the general life cycle of ich. Higher temps should be used in ich treatment which helps speed up the life cycle. And the ich is only vulnerable during the free swimming stage, not while on the fish or while a cyst in the gravel so speeding up the life cycle and exposing it to harmful substances (salt, chems) quicker will rid your tank quicker.
If you stop early the ich will survive and may be able to live in your tank for quite some time before you see another outbreak. This is why people sya that all aquariums have ich, they just didn't finish the treatment.
Using a gravel vac and doing water changes also helps to get rid of the ich cysts as well.