What should you put in your luggage when you are traveling?

Don't put anything with significant density--like a thick book or a large block of cheese (yes, people put cheese in their luggage). It's not that the book/cheese is suspicious, but they can't see through it to see what is under it, so they must open your bag--this is your fault, not theirs. Don't put anything that LOOKS like valuables in your checked luggage. Costume jewelry, cheap watch etc. A $25 fake watch may not be valuable to you, but it could look like a Rolex until stolen. And, while they are in your bag, they might snag that brand new $300 cashmere sweater still in its nice wrapper.
Don't overlap anything with a cord--even something you think looks obvious, like a hair dryer. Lay them out evenly, and separately, and band the cords (do not wrap them around the item) so that they can be easily recognized.
Don't have a lot of loose stuff in your bag. Organize your clothes and other items in organizers like "pack it" containers (super cheap at places like Target). Things of different density/materials show as different colors when x-rayed so don't mix things up--like things with like things. If you organize your bag and make your bag easier to X-ray you will probably have less likelihood of your bag being opened.
Again, nothing is going to insure that your bag won't get opened--by legit TSA or thieves. You can insure however, that you KNOW that your bag has been opened, so that before you leave the airport, you can check your bag and make sure everything is intact or make a report. That's the best you can do.