For trails, and just for riding I use my Theraflex pad. I was very nervous about buying one as they are pricey, but it has made a world of difference to my horse. It not only keeps the saddle in place most of the time (as much as a saddle pad can do), but it also is breathable, and fills in any weakness to the topline that my horses may have. I have both the western and the english one (I rarely ride English anymore, but I still have my english saddle and my english saddle pad).
I highly recommend the theraflex pad, I will never ride without one now they are amazing!