Agree with brighthair that I keep it short and sweet!
Some judges like you to walk away and trot back, but whether they do or not I trot (after walking away and trotting back if asked to), change the rein, ask for canter at the corner, change the rein through trot (usually asking for her to move on a bit), canter on the other rein, go round the back and gallop (the best bit), then somehow manage to stop, back to canter, then back to trot, halt, salute, and pat the pony.
I know what you mean Divasmum! As a fellow competitor I don't like it! Been at shows when people have been going for over five minutes, showing rein back, turn on the forehand, etc... It's a show, not a dressage test!
Walk away from the judge, trot, change rein across the diagonal, canter, change rein again, canter and then gallop in front of the judge along the top of the ring.
I gallop in front so that the judge can get a good view without having to peer through the other horses and also there's no chance of upsetting a horse as you aren't going past them.
You need to show walk, trot and canter on both reins. A figure 8 in trot and a fig 8 in canter with a canter extension. Don't go behind the line of other competitors.
Decide whether your horse is better at trot or canter and then you can work something out which will show off his best paces. I have used the above mentioned show and put down the line for not showing off my horse enough (?) - from 1st to 4th!!!!
Not to long, not too short - serpentines can be good as well because you get all of your trot work done on both reins, canter out of the top of the serpetine, change leg across the diagonal and extend down the long side. I also put a 2-3 steps of rein back in at the end before saluting to show obedience after a gallop.