I picked up a pair of polarized sunglasses at my local outfitter over the summer for $30. I don't know the brand, they're nothing fancy, but they are darn tough. When loading up my car to head back to school at the end of the summer I accidentally slammed them in the trunk lid. Half of them were in the trunk and half of them were out. I thought they were gone for good but I opened the trunk and they sprung back into shape. The only mark on them is a little nick on one of the arms. I was really surprised that's all the damage sustained.
I also water ski with them on, it helps cut the glare off of the water and I wouldn't feel too bad if I DID happen to fall and lose them since they could be cheaply and easily replaced.