Make sure you start out your hole as perfect as possible.
You only have controll over the first inch or so of depth, make sure it is perfectly centered.
Ensure the Drill is ground correctly use light pressure feeding it down.
Drill half the depth through, then repeat from the other side.
Once a Drill gets off center it gets progressively worse.
Run the drill all the way down from the second side.
Now pick best of the two sides and finish the hole through.