Personally, I don't like the whole tree, cone, drawings in the dirt, approach. I like to have barrels set up if I'm running barrels. Yes, it's good to practice turning around things out on the trail and such, but if I am teaching him a barrel pattern I prefer to have the pattern set up. Personally, I don't find a horse that is learning the barrels makes the connection between a tree and a barrel.
Like BuckOff suggested, you want to get control of him without the barrels before you add the barrels to the mix. Like she said you should have a perfect turn, you should also have a perfect pattern at the walk before you ask for it at the lope.
Ride the same pattern every time. Be in the same place at a walk as you would want to be at a gallop. You want him to learn where he will want to be. If he is hugging it too much, and you can't get him to pick up his shoulder and move his body to where it needs to be, he's going to hit the barrel.
Just some more to add to what BuckOff said. :)