Three tweaks to TPIR that really improved it (two in particular):
1. Going to an hour. This made the show more of an event and has, I think, contributed to its longevity.
2. Adding the Showcase Showdown (which, obviously, coincides with #1). This made it possible for a contestant who didn't win his pricing game, or won just a dinette instead of a car, still to get into the Showcase. It's still luck-based, much as the original system was (since being the top winner mostly depended on which prize you were fortunate enough to be offered in your pricing game), but a contestant's fate is in his own hands more than in the hands of the producer. Plus, it's probably the signature prop of the show now.
3. The Double Showcase Rule. Granted, this is the least of the three tweaks, but it still provides one important benefit (besides the excitement of seeing someone win both Showcases): It gives the second bidder an incentive to place a genuine bid even if he thinks that the first bidder has overbid. That makes the finish more exciting than it would be if the second bidder went for the $1 sucker punch.