Range finders are great pieces of technology. They help you quickly assess how far away your game is from your current position. They save you from having to guess distances and can give you the confidence that you need to make the shot. They are truly great pieces of machinery, but the fact of the matter is that we cannot all afford these devices or we prefer not to use them. If you fall into one of these categories, then you will need to figure out a better way to judge distances.
The first thing that you can do, if your state allows it, is to go stump shooting. This is not literally shooting at a stump, but rather you are shooting at a bush. In this case it would be beneficial for you to borrow a range finder to validate your guesses as you are shooting. In this scenario, you take shots at a target at different ranges and elevations so that you get comfortable shooting at those distances.
I want you folks to know that I own a rangefinder now, but there are a few tips and tricks that I learned along the way.
The way that I learned to judge distances was with my father. I was just getting into the sport and he would only let me shoot at ten yards. [Keep in mind that I was only 12] After months and months of shooting at ten yards, I had a pretty solid feel on exactly what ten yards was and how it felt like to shoot at those distances. Once I had that distance down it seemed easier for me to figure out what 30 and 40 yards were. I would suggest that you start out in this same way.
If you have any place near you that allows you to shoot at 3D Buck targets, then I suggest you try shooting there. This is one of the best ways to judge distances if you are bowhunting. I use this in conjunction with my ten yard method I learned growing up. If you know what the animal looks like at different distances, then you will be more confident when you make your guess on the distance. I find that it helps to notice the outline and size of the animal at different distances.
Keep in mind that (even with a rangefinder) you will always need to take into account elevation. If you have any other tips or tricks, please feel free to leave a comment.