Have one or more people hold the tree as upright as possible.
Hold a length of garden border rubber up to the tree trunk, or you may use an old garden hose or section of tire. Get an approximate measurement of half the circumference of the trunk and cut the rubber with a utility knife or reciprocating saw to fit.
Hold the cut section of rubber against the tree trunk and wrap a rope around the entire circumference of the trunk two to three times, situating the rope against the rubber to preserve the trunk. Once the rope is assisting in supporting the weight of the tree, the rubber will act as a barrier to keep the rope from cutting into the trunk.
Stretch the rope out away from the trunk on the side opposite the sag. Have someone hold the rope ends while you pound stakes 3- to 4-feet deep into the ground with a rubber mallet, angling the stakes away from the tree.
Wrap the rope around each stake and pull it taut. Secure the rope ends to the stakes by tying them around the stakes several times and knotting them off to hold in place. The tree should now be clamped or harnessed by the rope.