I have always been pretty good at judging distances. I sort of have to be in my work to drop a tree just short of an obtstacle, sometimes by inches.
I still practice judging distances when turkey hunting though. I hunt several states every year and will be in different terrains such as forests (both thin and thick), hilly country and fields. I actually judge paces not yards. Forty of my deliberate paces are just under 40 yards. As I walk through an area I am hunting I will pick an object and guess the range. Then I count my paces to that object to see how good I judged the range. Its pretty easy to practice as you do it while walking and I will do this several times a season, especially in new terrain. I also pace my kills off to see how far the gobbler was from me when I shot it as this also reaffirms my range judgement.
I agree that the closer you are to the ground the harder it is to judge distance. I try to pick a nearby tree or stump and judge the range before I sit down at a setup. When I judge range I do it in 10 yard increments, such as thats 10, thats 20, thats 30 and that stump is 35. If time allows I tend to reaffirm my range while sitting down and also will pick ranges at several angles as you never know where a turkey will show up.
Its not always possible to range objects before setting up so if a gobbler comes in and is near the edge of my range I will do the 10 yard increment thing out to the gobbler to make sure he is in my effective range.
For me the two most difficult times to range a gobbler is while I am in a prone position or across a steep draw or cut. In the prone position it is difficult because you can't see the ground much, but I still try to do the 10 yard increments. Across a steep draw is hard because there is no ground, just air. I again do the 10 yard increment but realizing that my estimated will be longer than the true range as the ground goes down then up but the shot goes straight across. I guess I just try to "straighten out" the range in my head and subtract the excess. In either case if I don't feel good about the shot its best to pass.
I also have my gun set up to be lethal farther than I intend to shoot.
I am pretty accurate at judging ranges out to about 70 yards. After that I am better offf with a range finder. I guess thats because I don't hunt with a rifle much, and I don't drop trees that tall either.