First thing is to avoid saddles with stiching/perforation, which just eats pants. I have no idea why saddle makers keep including this obvious and irritating flaw.

While i like my carhartts, the army surplus pants are really the best for riding- double thick seat, double knee, ties at the ankle and waist & reasonably priced. Hit them with some scotchguard/silicone water proofing, and you're good to go.