Cut out any old, dead or dying limbs, as well as anything that looks diseased in any way. This can be done at any time of year without damaging the tree.
Prune the tree for shape in the late summer. Remove less than one-fifth of the tree in one season.
Encourage a mushroom top on the tree by removing several of the base limbs. This helps keep the base of the tree open and encourages straight growth to the top of the tree.
Trim branches to encourage the tree to grow vertically and keep one central branch as its support for as long as you can.