Well, I start by using a cattle blower to blow the dirt and dust off my horse’s back. That way, there might be a little dirt but nothing to cake or be abrasive. This also helps a BUNCH with keeping the bottom layer saddle blanket clean.
The base layer is a $40 New Zealand Wool single layer saddle blanket, 32x30 or thereabouts. The wool allows sweat/moisture to wick up and evaporate. It is about 1/4" thick. With no abrasive dirt crusties, I only need to wash the blanket about once per year. I put it in a plastic bag, in a muck bucket, and drive around on the ranch roads for a day or so! If the blanket gets smelly, I just spray it with some Veterycin and that makes the stink go away.
The second layer is a wool felt pad. Somebody did a study of the concussion absorption of different types of fancy saddle pads, and the best shock absorbing properties were from a reindeer-fur felt pad. I don’t think you can buy a reindeer felt pad, but you CAN buy a wool one, and in thicknesses from 1/2" to 1" or more. The thickness you buy would depend on how your saddle fits a particular horse. I have a few different pads so my saddle can fit a few different horses’ backs. But anyway, if the saddle fits and the pad is an appropriate thickness, the wool felt does a great job of absorbing shock from roping, etc.
A couple of companies:
Diamond Wool:
http://www.statelinetack.com/item/diamond-wool-contour-cowboy-pad/E006941/
You can shim if needed with this pad:
http://www.statelinetack.com/item/diamond-wool-contour-relief-pad/E006939/
Really nice pads-Five Star wool pads- I buy them used via Craigslist or a consignment store:
http://5starequineproducts.com/saddle-pads/contour-horse-pad/
The difference in price has to do with the quality, and quantity, of virgin wool in the pad. Diamond makes a ‘more virgin’ version that costs what the Five Star pads do. I expect the Five Star pad to last longer than my $60 Diamond Wool pad.
If your saddle is a good fit, and you aren’t roping heavy cattle, a doubled (30x64") wool blanket would work fine, especially if it has wear leathers.
I have a friend who uses basic acrylic fleece cut out to match the contour of his wool felt pad, he washes these as you would a baby pad. He has good luck with this, but I’ve had horses that got terribly itchy from the acrylic fake fleece. Wool doesn’t have that effect.
I’ve heard nothing but bad reports for the fancy gel pads and such.