Proofide in spring, fall, and after it dries out if ridden in the rain. I also proofed the underside when it was new. It has never been left out in the rain, but has done some all day rides in the rain. I am a big fella and the saddle has about 12,000 miles of ride time on it. I guess the saddles that last 20 years realy are just garage ridden models!
O'well, seeing that nothing does last forever it's time to buy another.

On the issue of sagging. I lace my Brooks saddles. I have a leather punch that I use to punch 4 holes on each side of the skirt to lace the skirts and keep them from flairing. This adds a lot of stringth and firmness to the center of the saddle. I put the first hole just in front of the adjustment bolt on the seat post (do this after you have your for and aft established), then proceed twards the nose of the saddle leaving about 8mm between holes. The laces don't have to be pulled tight, just barly snugg.