Pull the moss off the tree with your hands. If necessary, stand on a ladder to remove moss in high spots on the tree. Moss should easily come off if you firmly rub your hand across it.
Screw a high-powered sprayer attachment on a garden hose if you can't reach some of the moss. Spray the moss with the water to knock it down. The closer you can get to the moss, the easier it will be to knock down. A power washer works even better for this, so use one if you have access to it. If you use a power washer, set it to a low pressure to ensure you don't cause damage to the tree.
Prune branches with a saw or pruning shears around the center of the tree to allow more sun to shine through. If necessary, climb the tree or stand on a ladder to get access to the branches. Once more sunlight hits the tree, moss is less likely to grow.
Repeat the moss-removal process as often as necessary. If you live in a heavily shaded area, you may need to use a commercial moss-killing spray to permanently take care of the moss. Be sure to follow the application instructions on the container's label for the best results if you choose to take this route.