To prepare your horse for riding or other activities, a lunge line can be used.
Things to Consider:
Young horses -- Lunge training can be very useful if started early on younger horses. Early introduction to the line and some brief lessons are all it takes.
Halters or Cavesson -- Both can be used to attach the lunge line, although the cavesson is a little better since there are more attachment points and they are better padded for comfort.
Preparation -- Before horses are put on a lunge line, they should be fairly obedient, calm, and be easy to control. You should also be able to lead them from either side.