How can I prevent cat litter boxes from tipping?

I'm now using wood shaving type litter in my litter boxes. I've encountered an unexpected problem - when the cats step out of the boxes with the wood shaving litter, the litter boxes tilt and spill much of the contents out onto the floor because the sawdust is so light compared to the clay pellet style litter. Is there any way to prevent the boxes from tipping up?