Also, make sure you're not bringing home sick ones to begin with. It's pretty widely known that most pet stores don't know how to properly care for chams, especially Petsmart and Petco. Make sure you look for the signs and pick out the healthy ones. Clear eyes, alert, curious, active, nice color, no lumps or bumps, straight legs, climbing, etc. - I think the signs are pretty obvious.
This is also a very good reason why you should buy from reputable breeders rather than pet stores. One, you're making the most out of your money and experience by getting a healthy cham. Two, purchasing from pet stores means that you just helped them make money off their cham. Come inventory time, they'll see that their cham made them money, and they'll restock more. That leads to more chams not properly cared for while at the pet store and when sold to ignorant buyers who are misinformed about their care. If the pet stores can't make money off of their chams, they'll stop selling them.