Assume the worst case - assume that you tent will get blown down in a rainstorm while you are gone. I always loosely stuff my sleeping bag (no compression) or at least roll it up in the sleeping pad and put it in the middle of the tent. This way if it rains and the tent leaks, your bag stays dry.

Assume small and large animals will check out your campsite. Take anything that smells of food out of the tent. Clean all pots and organize the cook area. Hang food or use a bear cannister. It is much more likely that animals or the weather will get your gear than people. Pack straps and trekking pole handles will have salt on them - Marmots love to chew on these!

If I plan on being in a very crowded area I simply take my old stuff. It is so horrible that nobody would want it! I have an entire set of "Yosemite Valley" basecamp gear.

If you are really worried, I have seen wire security mesh sacks. You then pack up all your gear and lock the mesh sack with gear and attach to a tree.

If I do a side-trip on a day's hike I take the food out of my pack and hang it and then hide my pack under bushes or hang it out of sight up in a tree. I never just leave my pack sitting out in the open at on a trail.

That said, I have NEVER had a thing stolen while on the trail. I have had plenty of critters chew on my gear! And I have had my completely packed backpack stolen out of my car while it was sitting in my garage (loss of nearly $1,000. Ouch!