Having a high quality saddle pad will really aid in your horses back staying healthy and comfortable. I would like to recommend my favorite saddle pads.
I use the Professional's Choice. These pads I use for everyday use, training, my students use for 4H shows or Local Level Shows, and trail rides.
This is a nice wool Pad I would recommend for showing . It's larger and a nicer design.
This pad is definitely worth the money. They last for years, and clean up real well. I shampoo and curry mine. Not only do they not slip, but they resist any pressure points on the horses back.
This pad is a lighter version, and the top is canvas not wool which makes it unlike the others. I like this pad for the summer. The same concept, just lighter and cooler. Measuring 30" X 32" it may be a close fit for some of the saddles with the larger skirts.
The last pad is a nice all purpose pad. The same concept for the English Rider.
Combine a good fitting saddle with any one of these saddle pads, and you will get a real comfortable ride for you and your horse.
One more thing I would like to add, you can check your horses back to be sure the pad and saddle are a proper fit by looking for dry spots after the work out. However, I don't always find that to be accurate.
The best thing to do is to run your fingers along the sides of your horses withers and spine, pressing down with some pressure. Watch your horses expression, if his head pops up or he gets irritated, he may have a sore back.
By having a properly fitting saddle and an orthopedic saddle pad you can greatly minimize the stress on his back. I'm sure there are other pads out there that do a good job, but I haven't tried them. I can honestly give you the thumbs up on the Professional's Choice Pads because I've used them for years. As I always say, "if it ain't broke I don't fix it, unless it's a horse".