When you are shooting uphill or downhill, try to maintain T form from the waist up. Bend at the hips and keep your arms perpendicular to your torso. Don't be tempted to move your bow arm independently of your upper body. That changes the dynamic forces on the bow, which could cause you to miss. It's much better to let the large muscles of your legs, hips, and waist control where your bow is pointed.
When you are shooting uphill or downhill, always try to find a flat place for your downhill foot because it will be carrying the majority of your weight. When you are shooting at a moderate angle uphill, shift your hips forward; when you are shooting at a moderate angle downhill, shift them backward.
For shooting steeply uphill, bend your uphill leg slightly. On a really steep uphill shot, you can actually kneel on your uphill knee. To shoot steeply downhill, bend your uphill knee and open up your stance to execute a better shot.