The biggest thing is finding a saddle that properly fits. Also making sure that the saddle pads, girth, or any other tack don't change the fit of the saddle.
Gel and memory foam saddle pads are really good. So are saddle pads with honey comb technology such as the supracor hunter/jumper cool grip saddle pad. It's one of the only saddle pads I've ever found to actually improve saddle fit. It has honeycomb air pockets, which collapse to contour between the horse and saddle.