We had a breakdown on safari in South Africa. Guides fumbling under hood, no radio or phone communication and we're stuck. Turns out there's something that needs to be 'secured' and they have nothing to secure it with. I have a zip tie. Smileys all around. It's not perfect but it should crutch us out of our jam. Now if we only had a second zip tie...well, actually, yes, I do.

I also keep some small colored ones around to put through the zipper pulls, mostly so I know if anyone's been in the bag.

I also usually carry a strip of webbing with buckle ends. It takes some of the pressure off the zips if my bag is particularly full and it can have other uses.