Longeing a pony is the process where you stand while your pony walks, trots, or canters around you in a circle. Its simplest use is for exercise, but it is also used for training and fine-tuning your pony. The most important aspect of longeing a pony is that it teaches him to pay attention to you. Thus, longeing is especially helpful with a pony that has problems taking things seriously. By its nature, the longeing setup focuses the pony's attention on you. Longeing also teaches a pony to relax and focus all his/her attention on you.
Longeing is typically done with a longe line (length varies, but 20-25 feet is most common). One end of the longeing line is attached to the pony's bit or bridle with the other end held by the trainer. It is not a bad idea to have a whip in case the pony wants to stop, or is otherwise not listening to you.
It's a good idea to start on a somewhat short length of line. Gradually, let some of the line out and step back, urging the pony to move around you in one direction or the other, and use the whip to cue him. It's a good idea to keep the longeing rein in the hand of the direction you wish your pony to go. For example, if you want your pony to go around you in a left circle, keep the longeing rein in your left hand, and the whip in your right hand. Then lead the pony around you in whichever direction you want him to go by dramatizing it with your hand, moving it in the correct direction and allowing the pony to see where you want him to go. As the pony moves around you in the gait you want, continue to let out the line until it reaches the length you want.
Indicate speed and pace with voice commands such as "walk on," "trot on," "canter," and "whoa." You can also click with your tongue to indicate to the pony you wish him to go faster. A flick of the whip on the pony's hindquarters can be used if the pony is lagging. Only a light contact should be kept on the rein. Pulling on the lunge rein is a signal to the pony to slow down or stop and face you. You can make the circle larger or smaller, by letting out or taking in the line. When you are ready to stop, move a bit forward of the line and say "whoa;" it is also a good idea to point the whip a little bit in front of the pony.
Longeing is a great tool because it can be used to teach the basics of each gait to a new pony, or to fine tune a highly trained pony. To help a pony get good balance and rhythm, you might consider introducing poles (start with one, then go to 2, and so on) on the ground which your pony must walk/trot/canter over while on line; you turn the poles into cavaletti once your pony gets the hang of things. In fact, you can even add a small jump or two for your pony to jump over on line.