I am anal about this. I got my first trailcam when I was about 15 and it was stolen the first time I put it out. Knock on wood, I haven't lost one since, but I take a lot of precautions. I like to think it's karma as I walk by about a dozen a year and the thought of nabbing one has never ever crossed my mind. I've only got one picture of another dude, early this spring, I guess he must have been shed hunting, and he just cheezed for it and moved on, so thanks dude!
Most important, far and away, make access difficult. If you put a camera up and think there's a good chance other people will walk by, there's a good chance it's going to walk away. No one sees it, no one steals it. There are some heavily used tracts of public you just don't put cams on, just common sense. Our family camp borders a heavily used gamelands in 2D and I would never put a camera there and expect it not to walk away. Scout the old fashioned way there.
There are some things you can do to hide it. Avoid the black straps, they stick out like a sore thumb. I buy brown cameras with brown straps, and I use a sharpie to DIY my own ASAT camo pattern on both. It doesn't make a difference if you walk right up on it, but back up to 15 yards or so and you would be surprised what a difference it makes. It disappears at 30 yards, you cannot see it.
I also will hide some cameras 8-10 feet up in the trees using climbing sticks or webbing aiders. I make 360 degree adjustable mounts from hardware store materials. Even there you want to pick the right tree so the profile doesn't just jump out at you, because the mount sticks out from the tree that can make it easier to spot. It's better to be well hidden at eye level than stick out like a sore thumb at 15 feet.
I also brush in my eye level cameras. A lot of times what I will do is lean a dead branch up against the tree, and with a little bit of brush make a little "nest" to hide the profile from the side. You have to be careful here...less is more...too much and it just makes it stick out even more. You have to make sure the brush you use isn't going to shift and block the lens also.
Some guys use locks, I avoid them. To me they just make the camera much more conspicuous, and if someone really wants the camera, they are going to get it regardless.