If you recall my last project, the Minecart Detection Entrance, this is a good way to get out. I suggest making this in your house/dungeon/castle/giant book/whatever you want to call it, because that will keep people from breaking into it on another mining run. Just remember that this is A ONE WAY DEAL. When you step out, you cannot go back in without breaking or going back through your other seccret entrance. Mind that this is only a companion to another entrance, and only suffices as a way out.
This mechanism is SO simple, you could probably just glance at the picture and get how to do it (Well, the second one -- the first doesn't really tell you anything).
- 2 sticky pistons
- 1 restone torch
- 1 redstone repeater
- 1 pressure plate
- Plenty of redstone
Step 1: Simple Connections
Find a spot that leads outside/into your hiding hole and stick two sticky pistons at the side two blocks away from where you want to exit (The room, that is). These pistons are going to be open until you tell them to close, so keep in mind you must hide the head/stick from sight.
Step 2: Wire It Up
If you found that a mouthful, just pay attention to my pictures here. The second one says a million words on its own.
Step 3: Finito
Thank you for reading, and remember -- you can't tie a button.