I borrowed a buddy of mine's rangefinder, climbed up in the tree, shot everything around me (within my range) and wrote it all down on a piece of paper for "record-keeping." That way I'll know the yardage when season comes and can leave the rangefinder at the house. (Keeps the pack lighter, and I don't own one anyways--it's a heck of a lot cheaper that way.)

Honestly though, I can usually estimate within close enough yardage, up to about 150. After that it get's tougher and my margin of error increases. I would say you only really need one for rifle hunting. Bowhunting you're usually close enough to the deer to estimate accurately.

Where you are, pick something out, say how many yards you think it is, and then step it off and see how close you are. It's kinda fun and it'll help your "eyesight yardage" get alot better.