Preferred Select is generally awarded to customers with Wizard #s who rent 10 or more times in one year (a trick though: they keep track by credit card, not Wizard #, for some odd reason; so you've got to have all your rentals on the same card).

They also grant Select to some airline's elite members in hopes of getting their service.

I do have to say, though, that Avis service generally doesn't compare to Hertz. I almost always have to ask for my Select upgrade and even then am often told it's based on availability. Not for me to say, but when you're looking at the window at 100's of cars it's hard to believe they can't bump you up one class... ;-)