The way I do it is points divided by age in hours (plus two) to the 1.25th power. This is probably not optimal, but it's simple.

 (= gravity* 1.25 timebase* 120)

 (def hotness (s (o gravity gravity*))
   (/ (- (realscore s) 1)
      (expt (/ (+ (story-age s) timebase*) 60) gravity)))

I avoid reranking all the stories by keeping them in sorted order, and whenever I generate a frontpage for someone logged in, reinserting one randomly chosen story near the top (where all the change is). I also reinsert any story that gets voted on.