I used to shoot competitive 3D archery, where life sized foam animal targets are shot at unknown distances. Each target had a set of scoring rings indented into the foam. Depending on which ring you hit, you could score a 5, 8, 10, 12, or 14. Hit the target, get a 5, hit the vitals, get an 8, a clean kill shot was 10, and then the 12 and 14 rings were put in risk/reward type places. In order to hit the 12 and 14 rings, you had to be able to judge yardage accurately to 1 yard. The best guys I ever shot with could judge accurately to 1/2 a yard, and aim at and hit those bonus rings at 50 yards. It was unreal. The best I ever got was being able to judge within 3 yards, out to 45 or so, then I was good to the nearest 5 yard mark.
One of the ways I used to practice was to play a game of "HORSE" of sorts with my archery buddies. We would set up a few targets, then call the spot to shoot from. We would usually end up pushing the yardage pretty far. So one target I call, thinking its about 47 yards. I make a good shot, and end up a bit high. One of my buddies steps up, aims, and hits the 12 ring, dead center. Looks up at me, grins, and says 45.5 yards. Another buddy ranged it with his rangefinder. Sure enough, it was 45.5 yards. It always amazed me how good that guy was.
So if you want to get really good at yardage, talk to some serious 3D archery competitors. Of course, there really isn't a secret to their ability. They have access to the targets that are used on the competitive circuits (they are standardized) and spend tons of time looking at them, guessing their yardage, and then lasering each one to confirm.
A good idea may be to convince your BP club or shooting/hunting buddies to purchase a 3D deer target, not to shoot, but to use to practice judging. You could even judge it, then aim your rifle at it, so you get an idea of sight picture at different distances. If you really wanted to go all out, you could put some priming powder in your pan, and touch it off while aiming at the deer target.
If that is out of the question, just bring a rangefinder with you everywhere you go, and try to judge the distance to everything you see. Guess, and laser it to confirm. You'll be amazed at how quickly you get good at judging distance.