While the best security is often stenography aka security through obscurity, it becomes irrelevant if someone knows what to look for or how. It doesn't matter if you keep your gun in a book if they know it's in a book at all. Similar rules apply that if something well hidden is known about to begin with that whatever is used to hide it becomes irrelevant. Security is only as strong as its weakest link which becomes a downfall of supposedly impenetrable security systems. What good is a biometric scanner if all it really takes to open the door is two wires?
The best method I've found in designing security is like a bomb you never want to be defused, something designed to never be bypassed. Essentially you're designing it so that if the wrong person ever tries to get through then nobody ever will again. Multiple stages, lots of redundancy and essentially barriers to entry. Take for example some of the newer biometric scanners. They can scan a wide array of things and then interpret them to a series of electrical signals and thus if the sequence is out of order or at the wrong frequency the locks won't disengage and the door remains shut. Simply put the best security is the one that makes it hard for even you to get through.
I apply this principal with my digital files of extremely sensitive nature. I encrypt my drive then encrypt each folder and or file as an extra layer of security. For important files I split them and hide them inside other files, making it so that even if you find that some files are hidden recovering all the parts becomes very difficult. I had to devise an entire procedure to remember how to formulate a password for a given file, meaning that no two files use the same password, yet another layer of security. For some of my most absurdly secure files it can take almost an hour to recover them. Though obviously this isn't something you do on a regular basis, these are the high level things you don't want anyone to ever have access to and it's a small price to pay for that kind of security.
For those of you more interested in keeping your wallet safe than your hard drive there's plenty of options out there. There's hidden wall plates you can buy that flip out making it a good place to store something as nobody thinks to check the outlets or switches in the house and even so they'd have to pull the plates off to know for sure if anything is there from the existing wires. This works better than just behind the wall as then you have to do drywall every time you want to get to it and if it's metal it stands out with a metal detector. The concept to this is that anybody that knows what they're doing is going to check under your mattress but they might not check inside it, if it's harder for you to get to then it's harder for them to get to and the harder for you the substantially harder for them. Remember though that it doesn't matter how clever you are if they know where it is too. That's why I like things like a false bottom to a floor safe and then an additional safe under it for the real storage. This added level of security makes it so time consuming it loses value to whoever is trying to get what you have and that's only if they don't give up after finding your first safe empty or only minor value things. I like to keep a few dollars and maybe an important looking file of garbage in the first safe so they don't suspect another safe and they feel like they walk away with something worth the effort. This way they're less inclined to come back.
So a quick recap. Redundancy, obscurity, small rewards, and increased difficulty and time are important parts in designing a security system to keep people out as long as possible or convince them to quit early. Combined effectively the chance anyone gets to what you really care about becomes smaller and smaller and the length of time required to get to it increases making their overall chance of success drop substantially to a point in which it becomes more likely they'll be interrupted or discovered. Once they've been found out what they were after becomes irrelevant to them wanting to get away in most cases.