The best area to do this is at the start of the game. There are three barrels in the room where the game begins. Get two of them and stack them together. Drop one barrel on the ground by pressing X to drop it. Then, take the second barrel and drop it on the first one. When both barrels are stacked, lift them up by pressing Circle. Next, walk in a straight line for 10 meters to get the "Balancing Barrels" trophy. Walk slowly and do not turn around. The starting room is great for this because there is a lot of space and you can go there at any time by starting a new game.