I like to use the analogy of the brain as a muscle with DS. I tell DS that yes, he's smart, but in order to stay smart he has to work his brain just like he would have to work his muscles in order to stay strong. If he's not making any mistakes, it's not a hard enough "workout". As Kriston mentioned, this doesn't count the careless mistakes that comes from rushing through the (way) too easy math homework he gets. I make sure not to make a big deal out of those types of mistakes. He just has to go back and find the mistakes and correct them - but I stay very nonchalant about them.
