You can also use nylon ties which cost only pennies and the TSA can cut if needs be, but would deter an opportunistic thief. Another solution is to use a combination lock, but affix a luggage tag to your bag with the code on it! Once again, the TSA can get in if they want, but the thief won't want to mess about with twiddling the combination.

FWIW, I would also advocate using a twist lock or small padlock on the bit of your cabin luggage where you keep your passport, wallet etc. (Honestly! You don't keep these in the seat pocket in front of you, do you? ) These items can be filched in seconds from an insecure bag in the overhead bin or from your seat whilst you are either dozing or answering nature's call!