disengaging cavalry...

[caravel]

When charging I always single click. Never have a unit run into a charge. The unit will automatically go into a charge when it gets to the right distance it doesn't need to be running. If you run it in, it may end up too close and not in good order which means that it may not charge at all, but run up to the enemy and start meleeing. The best way to check that a unit is charging is to keep the mouse over them. If the unit is reported as "attacking" it's charging, if it's reported as "fighting" that means it's meleeing. Once the charge has lost it's oomph and the kill rate has dropped it is then a good idea to withdraw it, as described by Roark above and bring the unit around for another charge.

You can also keep attacking/pursuing units formed by keeping them in hold position until they make contact with the enemy then switch them back. This helps the unit stay together and stops it getting fragmented.