FPS crouching option

How do I make the player crouch?
I really need this option for my game. I can make the player crouch in a 3rd person game, but FPS is different.
Please help me.

ps: I prefer logic bricks.

why is that different? The main difference is the camera position.

I advice to search for crouch or crouching. You can even look for FPS templates. THe most of them have that implemented as far as I know.