If you don't want to watch a video you can read the tutorial here.

Stand on your chest and enter the command below... Somehow I managed to make that into a 2 minute video...

It also works with other storage blocks like hoppers or beacons.
If you rename the block you are locking, it will say that it's locked instead! (A chest renamed "Party Chest" and locked would say "Party Chest is locked!"

Command for locking: /blockdata ~ ~-1 ~ {Lock:"KEYNAMEHERE"} where KEYNAMEHERE is the name of the key you would use to open it.