I'm not sure how much I can comment, I am slowly getting the impression that security is a far greater issue in the US than in Australia/NZ/Europe/SEAsia, but anyway:
- D / U locks are good, but only if you have a narrow secure metal thing to attach to. Otherwise a cable lock or a (hardened) chain are more versatile - tree, light pole, etc.
- Take panniers into the tent with you at night (or in the vestibule, out of sight)
I guess leaving stuff is the time of greatest risk, what to do depends on the situation. I'd feel fine leaving a tent up in the middle of the bush, but probably not in a campground. I guess this is the "stealth" camping people here talk about.