One solution is to wear different breeches or, cleaning the saddle with saddle soap which leaves a bit of a tacky residue behind.
What you do if you use saddle soap on a saddle is to squeeze as much water out of the sponge as you can, then load the sponge up on a bar of saddle soap (don't use the squishy stuff sold in tins), and then lightly cleaning the saddle. You then allow the saddle to dry for about 15 minutes and lightly buff the saddle with a towel. The residual saddle soap will tack up slick areas a bit.
Way-back-when the leather in saddles was really high quality, they used to stiffen the leather by infusing it with hardened bee's wax. Today they use paraffinic wax which almost never leeches out of the leather.
They way they used to get the wax out of the top layers of the leather many years ago (and I don't recommend this process on today's leather), was to use ammonia water on a rag (only dampened, not dripping) to take the artificial wax finish off the top of the leather. It made the saddle surface look like hell in some cases, but it did de-slick the hard 'factory finish. Some hardcore old-time foxhunters still use this process on their saddles (just to make their saddles look like they've been used for years when they weren't
), but the saddle has to be made in a certain old-fashioned way with really good leather for it to properly work (which is why I don't recommend this ammonia process as it can really screw up anything but the extremely highest quality leather if you make a mistake in the process).