First find the place you want to hide, for this I'm going to be using the entrance to a small fort I built.
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Next fill up the are with whatever material and leave 2 blocks of space for a door.
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Next, pick a door! I used Iron as I feel it is better then wood, although that is just me personally. It really doesn't matter what door you use.
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Next put a painting over it, I know it will look weird on the fort, but in your house it will look like any decoration.
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Next put something to turn on the door, it can be anything. For this I will use a wooden pressure plate but I have used buttons, levers, and redstone currents in the past.
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You should be able to walk through the paining! Be warned if the door is open long enough the painting will drop. Make sure you have a way of getting out. This is how mine looked like. [ Register or Signin to view external links. ]

Other examples I have used:
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*Note It can be easily found, I usually when using a button make sure it blends and then destroy it and put it in a chest after I'm done to make sure no one randomly walks into it.