I've had pretty good luck with a 7/8" Diversibit and sliding a piece of 1/2" EMT over the length of the bit's shank. The EMT keeps the bit from flexing too much. By lying the post down on it's side for the drilling, you can keep a better watch on any wandering that is happening and stop it before it is too late. It is also best to set up some form of a jig so that the drill motor will be positioned at a height that is centered.
Oh, and you should drill a starter hole that is about 6" deep before using the long bit. Trying to get a long bit started is easier if there's a pocket to hold the tip straight.
