In the western mountains, bowhunters just don't get very many shots on flat terrain. Shooting at steep angles is tough, because you can't just range the animal and shoot. You have to figure out where to aim.
Although it may not seem logical, you have to aim lower on both uphill and downhill shots than you would on flat ground at the same distance because your arrow doesn't drop as much at a given distance when it is going uphill or downhill. The rule of thumb is to aim for the horizontal distance to the target rather than the true distance.
Intuitively, this makes sense on downhill shots, because you're shooting the same direction as gravity is pulling your arrow. However, the principle is a little harder to grasp on the uphill shots. Nonetheless, that's how it works. So remember, anytime you shoot uphill or downhill, you have to aim low.
Now you can guess how many yards to cut, but that's just what it is -- a guess. The compass I always keep in my pocket has a little clinometer attached, and I use that to determine the angle to the target. I then consult a laminated chart I have made up to tell me how much yardage to cut from each angle and distance. Of course, these days you can buy an angle-compensated rangefinder that automatically makes these calculations for you.