The Ender Chest is the best approach, but if you don't want/can't use an Ender chest there, here are some solutions I have prevented theft in Minecraft.

Redstone Traps

I have in the past created a storage room with tons of trapped chests, ~25% of which were "real" and the other ~75% killed everyone in the room. Fall damage is the best in my opinion, because it is hard to prevent once that trap is triggered, enables you to collect the thief's items, and your own items aren't destroyed if you trigger your own trap. Lava takes time, and the thief can break/get out, and his items are destroyed. Don't use TNT because that destroys your items. This usually only works once though, and trapped chests are suspicious. I like this approach because I am a Redstone kind of person, and I like the challenge of creating red stone traps for the thieves.

Hiding Your Chests

Obtain stone blocks. Go down into your mine. Mine 5 blocks down. Place chests and valuables. Write down coordinates. Place 5 stone blocks above. Tell no one that you even hid a chest. This method has always worked for me, and it is cheap.

Having the item in your inventory

Items in your inventory can't be stolen when you are offline. They can kill you, but you will know who did it, and can talk to them/take revenge depending on your situation. I like storing all my valuable diamond armor on me. Swords too.

As a side note, I usually never have all my diamonds in one spot. I distribute my diamonds among chests, and when a thief is successful, they usually leave after they steal one set of diamonds.