On offense, they don't have post players, so if they're jumpshooting is off, you can get them off their game. They have solid ball movement, but pressing the top and elbows and forcing them to make passes inside can result in some stops and some turnovers.
On defense, they start two point guards. While Terry is a decent defender and Devin Harris is a very good defender, they are both about 6-foot-2 and slender. Having a big and/or strong off-guard that can exploit one of them could force them to change their defensive approach.
They shoot a lot of jumpers in transition and delayed transition opportunities. Getting back on defense in transition is important. Then when the halfcourt is set, force the ball in the middle. Force the ball out of Dirk's hands and make Josh Howard the main facilitator, and force him to take a lot of jumpers. He'll make a lot of them, but then he gets into the mindset of shooting every time down.
Those are a few things ...
But, the Mavs are very, very good. Just a few things that I would attack if I were coaching against them.