I agree with what DR said about leaving the clutch a little loose so wire can pull off if needed. I can usually always free a snagged ball by either backing down or circling around and pulling the opposite direction I was trolling. The other little thing I've done to save gear is use a piece of 100lb mono that I hook between the downrigger ball and the cable end. If the snag is real bad the mono will break and you lose your downrigger ball but not all your other gear as well.
I've found that a good pair of dykes will always cut my cables just fine and I keep one hanging in the back of the boat.