Measure twice, shoot once. Or better still, measure thrice or "quadrice"...

These are the 4 ways I measure range for hunting/shooting.

#1 Pre-hunt ranging & Marking For an ambush style set or stand, before the hunt, I'll range out different landmarks and record the field on a map. If landmarks don't exist, I'll even put up range flags around the field to remain throughout season. Obviously this isn't possible for stalk or still-hunt type hunts.

#2 Eye Balling It's worth while to spend time training your eye to recognize ranges, especially in hilly territory. Our brain estimates range naturally by measuring how much land it sees between us and the target, which means our brain is easily fooled by looking over hills or valleys. So our instinct can be improved by paying attention to what certain common elements look like at different ranges - paying attention to the relative height of fence posts, height and width of trees and tree trunks, resolution of grasses, branches, tree leaves, etc. Of course, being able to tell the difference between 800 and 850yrds, or even 800 and 900yrds with the naked eye is rather difficult - the accuracy of the instrument just isn't there.

#3 Reticle Ranging I use Mil-dot scopes primarily, but I keep note of the subtensions for all of my scopes so I can use the reticle to determine range. Like Flags mentioned, this changes with different magnification setting - for second focal plane scopes at least - so it pays to practice with your scopes at known distances with known common objects. I don't like to use game animals for my primary reference - with the exception of relatively short ranges - because variation in body size means variation in range if you assume the same common size. Huge step up from eyeballing in terms of accuracy.

#4 Laser or optical Rangefinder It's not the end-all-be-all, but it's more accurate than any other method, without question. At longer ranges where being off by 10yrds can mean the difference in wounding/crippling game, but not quickly killing it or even a complete miss, this makes a lot more difference than it does at shorter ranges where 50yrds might only mean 2" of change in POI.