Due to the unavoidable problem of stealing that occurs on all Minecraft servers we have a chest protection plugin called Lockette installed. This does not protect only chests - it can do doors, dispensers and furnaces too.
The great thing about Lockette is it is so simple to use. Simply place a sign next to the chest you wish to protect and put [Private] on the first line. This picture explains what to do:
Additionally, you can enter [Everyone] on lines 3 or 4 instead of a user name to allow everyone access to your chest. To make a chest only available to VIP's add [VIP] on the 3rd or 4th line, or [vipgold] for VIP+.
You can add more users by placing additional signs beside the chest with the 1st line as [More Users], where lines 2-4 specify the names of the other players.
If you are thinking 'I DON'T UNDERSTAND THAT, WHAT DO I DO', this video makes it very obvious:
If you forget all of this, type /chest in-game for a quick re-cap.