Master Lock currently sells a card of three 6' cable locks that are keyed alike. I didn't bother to weigh each lock but they don't weigh much. These locks are plenty long and sturdy to prevent your bike and gear from wandering away if you don't have your bike within eyesight at all times. I had two of them with me on my last trip; one to take care of rear wheel and panniers, the other to take care of front wheel and panniers. Two thin cables are better than one thick cable, because they make it a lot easier to lock up everything on your bike. I store the cables locked around the nylon webbing handles of my panniers so that the cables are easy to access.

You do need to realize that you still need to be within eyesight of your stuff if at all possible and to keep all your must-have stuff on your person. Locks just make it slightly harder for thieves to take all of your stuff by wheeling away your bike.