Trial and error and patience. I've taken a lot of these off before from different cars, but these were the absolute worst by far. Heat didn't work at all, alcohol didn't work at all either.
After plying off some with those methods (and kind of marring up the passenger side) I found that Goof-off (not goo-gone, two totally different products) kind of worked.
One of the things that I found was that the ink on the sticker prevents any sort of penetration by the liquid. So first step was to soak a towel in goof-off then let it sit on the sticker for a good 10-15 minutes. That will saturate the sticker (it also saturates the plastic and makes it all baggy, but it tightens right back up after about 24 hours of being dry). Then I took a heavy duty Scotch-Brite kitchen pad and rubbed off as much of the ink as I could (especially the yellow stuff at the top). Repeated a number of times. Then I completely resoaked and just started picking it off with my fingernails (credit card was too thick). It would come off in roughly dime sized chunks and it still left a lot of adhesive behind. I had to constantly respray to keep it moist enough.
When it was finally clear of all the sticker I attacked all the left over adhesive by just spraying it down with goof off and scrubbing the crap out of it with a towel. This also took forever because whatever they used for adhesive didn't want to come off and spread all over the place. Eventually though (like 45 minutes per visor) 99% of it was gone. I then used copious vinyl protectant and armor-all and it looks pretty ok.
It's certainly not perfect as you can still see a faint outline of where the sticker was and in some of the places where I had to scrub especially hard it completely smoothed the texturing. The smoothing is much worse in the areas where I was experimenting, but it's still like this in some of the places where I used the above method.
Oh and word of warning, Goof-off will corrode and stain the plastic piece that goes over the mirror (learned that when I set a mostly dry towel on one...oops). So don't do that and mask it off if you're not going to be extremely careful.
So after a couple hours I ended up with slightly damaged/worn visors instead of stickers. It was worth it to me (I can't stand the bright stickers), but I wouldn't necessarily recommend it. YMMV.