If it's a lower branch that can be removed without negatively affecting the look of the tree, you might consider removing the branch, especially if it is attached at a narrow angle, creating a V-shape crotch. Otherwise, you should consult a certified arborist to see what might be done. Small splits can sometimes repair themselves if they're not subject to continual pressure. Larger splits--especially on branches that are under undue stress--may need to be supported with bolts. For a certified arborist, go to www.isa-arbor.com, the International Society of Arboriculture.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine