the most important part of preventing your car from smelling like ass its to keep it clean!! That means no old soda bottles and dirty clothes... that also means vacuuming, dusting, wiping and airing out your car everyonce in a while. I hate it went a persons car becomes soo cluttered to the point where you can't even sit comfortably in the back seat, and when there is no trunk space.
As for air fresheners, I persoanlly like vanillorama... but whatever you choose, stick to the same type.