The plastic cable ties are the most popular. I use them instead of locks. They will keep your luggage sealed and also let you know if your bags have been opened. Keep a nail clippper and extra cable ties in the outside zipper pocket of the luggage. This way you will have the clipper to snip the ties to open your luggage and extra ties to use when you are ready for your return.
Here is a picture of the cable ties. They are the same as you would use to organize cables or wires. You can buy them at any hardware store or a Wal-Mart or K-Mart type store.