i use a magic tree but dont hang it on the mirror, i hate stuff there, i hang it off the rear passenger handle on the roof, cant see it there.
and as said only cut the V and do it with sissors. then just pull a little more out every coupe of weeks, they last ages then.

when i worked at bmw the valeters used to spray a bubblegum air freshener onto the rear footwell mats, used to work well.