Reduce the weight on the branch that is splitting from the crotch by pruning a third of its foliage away with pruning shears. If the split is low on the trunk of the tree, prune upper branches from the weaker side.
Raise the weaker side of the tree to join the sides of the split together. If the tree is large, use a block and tackle. If the limb is small, lift and secure with cable or a length of clothesline. Think of it as putting a broken arm in a sling.
Get several long, threaded brace rods with nuts for either end at the hardware store. Drill a hole through both sides of the split using a drill bit that is 1/16th-inch larger in diameter than the rod. If the split is deep, install more than one brace rod.
Install the rod, tapping it through the hole with a hammer. Cap both ends with nuts, and screw them down tight with a wrench.
Carefully cut away loose or torn bark from the sides of the split with a sharp knife. Seal with pruning paint to prevent the bark from drying out over the next few months as the split seals and heals by itself.