I don't claim to be great at judging distances, but here's how I do it:

I sit in a different tree almost every time I go into the woods. After I get my sticks and tree stand set up, I go out to what I think is my best shooting lane opportunity and pick a landmark tree/bush, etc. Then I pace to the tree where I'm set up. Sometimes I do this for a couple of what I think will be my most likely productive shot spots. Then I get up in the tree and the first thing I'm doing when it's light enough is picking out those landmarks and slowly turnign my head, mentally making a circle around my stand at those distances and seeing other landmarks. For some reason I find that, once I have a good bead on a specific distance, it's easier to just draw a circle (mentally) around my stand and I feel much more confident in my distance judgment.

I should probably also point out that I am fanatical about scent control, so I haven't had problems with deer scenting me along my "pacing paths". Also, I won't shoot at a deer that is more than about 25 yards from me. And I generally stay within 20 yards, unless it's just a perfect shot at a relaxed deer. There are just too many twigs, etc. in the woods I might not see if they were further than that, and I really don't want to wound a deer unless I kill it. I mention this because it means that my distance estimates are going to be fewer and simpler than people who shoot out to 30 or 40 yards.