Ok, this is very easy, and is a lot easier as a rogue.
If you see a chest, and other group members didnt, dont go to open it. Wait until you have finished the instance and others have left. Now go back to the chest and loot.
Since 1 chest is worth 30- ?? gold, this is very nice income, just incase you're lucky and nobody else goes for that chest.
Rogue: If there's a locked chest, just say that you cannot open it. Like said above, after people have left, just go and have your loots.