Your reins should create a straight line from your elbow to your horse's mouth.
When the reins get too long, your hands are either too low or they get up way too high. You lose your leverage, your guide and effectiveness.
Your reins are a steering wheel. A horse carries most of his weight in his front end, so most of your guide and your balance is in the front.
You want light contact on the horse's mouth and drive from his hind end.
Use your leg as your gas pedal to control how much drive you want. Keep an even amount of pressure on your horse, and do not move your legs back and forth.
I have kids who ask me all the time, "Are my reins short enough?" Yeah, standing there they are, but in the pen when you're moving you have to feel and be able to adjust your reins for whatever maneuver you're performing at the time.
Whether you are showing in pleasure, horsemanship or western riding, there's no set amount of pressure. It's give and take.
Release when your horse gives, and take hold when he takes hold. If you take hold of a horse and feel that he has not given to you, and you pull just a little bit more, that's probably enough pressure
Just be patient and keep working. If you jerk on a horse, he is going to be resentful to that much pressure all at once. There is not an actual pound or amount of pressure that you need to pull. You just have to feel whether the horse is giving to you or not. When you take hold and feel him giving at the bar and in the poll, you have pulled enough. If you pull really hard and feel your horse pull back, you have over-pulled.
Of course, horses are individuals, just like people, and some are going to take less pressure, and some are going to take more. What a rider has to learn is to find that happy compromise in between. Horses that have been overtrained or mistrained are sometimes a little harder because they already learned an incorrect way. Sometimes you need to go back and just use a snaffle bit. Do a lot of side to side work, and use different pressure points. If that doesn't work, go to a gag bit where you put more pressure on the poll instead of the mouth. The most important thing to remember is to trust your horse and trust yourself with a less severe bit.
Article from AQHA's America's Horse Daily.