According to a post from The Horse of Course: "Finding the right bit can be tough. We're here to help. We've been in the tack industry for a long time, and we've partnered with bit engineers, bit developers, and top trainers and riders in our quest to find the best bit for you and your horse."
Get the right size.
Many riders are using bits that are too big for their horses' mouths. If you feel the underside of your horse's head behind the chin groove, you'll notice that the bones aren't very far apart. Those bones lie under the bars of your horse's mouth- where the bit sits! Most horses have a fairly small oral cavity and don't need bits much larger than 4 3/4" for a curb and 5 1/4" for a loose ring snaffle.
It's also important to make sure your bit isn't too thick for your horse's mouth. If your horse has a shallow palate or a big tongue, a thinner bit may be more comfortable for her.
Get the right material
A good bit should encourage chewing activity by oxidation. Copper is a popular, easy to find choice. Look for an alloy that is at least 70% copper- it will warm to the temperature of the horse's mouth quickly and promote the production of saliva and chewing activity.
Think about how your horse takes the contact
Is your horse heavy or light? Is he consistent in the contact? Assessing how your horse takes the contact is a key component in finding the right bit. A new bit can't cure training problems, but it can help your horse be more comfortable and understand your aids better.
Check equipment rules for your sport.
It's essential to make sure the bit you want to use is legal for your sport (if you're planning to compete). The national governing body for equestrian sport, US Equestrian, has online rule books available for perusal. Rules change regularly as sports evolve, so it's always a good idea to review them before competing.
Listen to your horse.
There's some element of trial and error involved in finding the right bit for your horse. Your horse is unique, and it's important to listen to the feedback she gives you.