Having the right bit on each horse is crucial. You have to experiment to find the right bit for each horse, and the better horseman you become the better feel you'll get for each horse's mouth.
When it comes to proper fit of a bridle, you need to consider not only the length of the headstall, but the proper adjustment of the curb strap. Generally speaking, I like a bit to be snug in a horse's mouth. I don't want a bit to be hanging too loose in a horse's mouth or it'll be completely ineffective.
If a bit is too tight or too severe for that particular horse, he'll have a tendency to throw his head up or gap his mouth open when you take ahold of him. If it's really bad, he might even rear.
If you've got too light of a bridle on a horse, you have no control and you're going to have to try to rely on your strength to get him slowed down and stopped. I don't know many people who are going to win a pulling contest against a horse.
The curb strap affects the way a bridle works, too. If it's too loose, the horse doesn't respond to it. It's almost like riding with a halter. If it's too tight, you make the bit a lot more severe and if you use too much pressure on the reins you can make your horse rear up. The general rule of thumb is that you can get two fingers between the horse's jaw and the curb strap.
Every roper should have a huge selection of bits. Over time, you'll develop a feel for which ones work best for different kinds of horses. What you're looking for is the bit that's only as severe as is effective, where you feel like you have enough control with the least amount of pressure.
The adjustment of the length of the tiedown strap is crucial, whether it's a mild, flat leather one or a more severe rawhide one. If your tiedown's too loose it's useless. Your horse can throw his head in the air and run off. If it's too tight, you aren't going to get the maximum run out of your horse and he'll likely want to cheat you. You want to adjust the tiedown strap to where your horse has a natural headset.
Where that tiedown hits the horse's nose is important, too. You don't want it too low, because that area's so tender and can cut a horse's air off. If it's too high there's not a lot of feeling there, so it won't be very effective. Ideally, you want it in the middle of the nose
You need to be sure you snap a tiedown right between the horse's front legs so you don't rub him raw. Sometimes after you've made a few runs you'll have to tighten your cinch some more, so you always need to be checking your adjustments.
The right amount of pads under your saddle is also important. If you overpad your horse, your saddle will tend to roll when you take ahold
If you underpad your horse and your saddle's sitting right down on his withers, it's going to be putting a lot of pressure on them and your horse is going to get sore. That's going to affect his performance. He might buck or quit pulling.
You should be able to stick your hand between the front of the saddle and your pads. My favorite pad combination is a Navajo blanket and a closed-cell foam pad. I like to use the closed-cell foam pad next to the horse so the sweat runs out from under it.