If you're travelling without a lock for your backpack, try this simple method to secure the zipper. Just pull the tab of one zipper through the other, then the second one back through the first. The end result keeps stray hands out of your gear.
The video above shows you how it's done (the it's a short video, but the action starts at 0:43 for the impatient). While it's not foolproof, it will definitely make sure that your double-zippered backpack or camera bag is pulled shut and secured from roving hands -- at least secure enough that if someone tries to get into it, you'll know immediately. It's not perfect, and it's better to have a proper padlock that keeps both zippers in place and shut, but this is a good option if you don't have one.
It's important to note that this is only helpful if you're actually wearing your bag (and you're paying attention to your surroundings). After all, if someone really wants to get into your bag, they don't even need to use the zipper, padlock or not.
Quick Tip: Secure Your Camera Zippers Without A Padlock [DIY Photography]