If you're consistently renting 10x/month, every month, I'd start working my way through all the programs, for their respective Elite levels. Believe Hertz ia 10x for 5*, and 40x for PC, so that's a start.

You could also wait out the programs, as there always appears to be one having an RDM special. The Avis/DL 1day/3000rdm 3-5/day/5000rpm comes to mind (some reporing 100k+ rdm returns). I'm a Hertz GC 5*, yet if I had had the time, I really would have dipped into that Avis promotion.