IMNSHO, about the only fixes, leather being what it is, are re-covering with suede (blue recommended for Elvis fans), which has its problems, or finding different riding gear with grippy pants. Tank knee pads aren't a bad idea, but 200 miles of knee gripping gets old fast. I suppose Plan C is a saddle with enough shaping to discourage sliding. OTOH that doesn't leave much opportunity to shift positions on a long ride.
Some days you're the dog, some days you're the tree.
Some days you're the dog, some days you're the tree.