Master the angles.
Regardless of shooting angle, you should always aim for the horizontal (level) distance to the target. In mathematics, this is called the cosine of the shot. Unfortunately, the cosine is often impossible to judge by eye, so here are some practical solutions.
From a tree, you have three options. A correctly adjusted pendulum bowsight rotates with shooting angle, automatically compensating for downward shots out to 30 or 35 yards. However, pendulum sights work only if your stand tree sits on level ground. Over sloping terrain, the pendulum principle breaks down completely.
A second tree option over flat terrain is sighting in your bow from an average stand height of 20 or 25 feet. Use a laser rangefinder to determine ranges like 20 and 30 yards, and then adjust your sights for those ranges. This compensates for downward angle, allowing you to use a laser rangefinder directly on deer without the need to aim low on the target.
The third, and best, angle-shooting solution for both treestanders and mountain hunters is the new generation of angle-compensating laser rangefinders. These units automatically compensate for shooting angle in any situation. Sight-in your bow at ground level, use an angle-compensating rangefinder, and aim dead-on!