I actually used a golf distance finder from the golf shop. It is only about $30, but does the job for bow hunting.
That's a great idea ATC. I don't chase the little white balls around yet, so I wouldn't have thought of it. Very good tip.
I find the best way to learn estimating is to shoot 3D.
Either use a rangefinder and burn the images of the different distances in your mind, or go without the equipment and make note of guesses that are off. Pace it off when you get your arrows back.
Than practice from your stand if you can. Things look further away from up there and may cause you to aim a bit high.