>No redstone/pistons visible, fully hidden. 1 wide, 2 tall. This can be made into a 2×2 door, can be copied to the other side to hide the door from both sides. There are many possibilities.
0:15 – Tutorial
05:35 – Fixing the bug when the contraption doesn’t hide the piston when used with a lever as the input.
07:00 – Adding a T Flip-Flop to the design to use a button as a lever.
ABBA switch – http://www.minecraftwiki.net/wiki/Redstone_Circuits#ABBA_Switch