Tutorials/Secret door

Using Lava
The most simple way to build a secret door is to use Lava. You have to build a frame, place ladders next to it (to stop the lava) and place some rails with a minecart behind it. Then you're done.

Here's a video tutorial of building that type of secret door.

Secret Painting Door
Start by making a 1x2 gap where you want your doorway to be and place signs down the sides of it. Then you place paintings on the bottom sign until it covers the hole. You may place a door in the hole.