how do i stick HOV decals using magnets?

Find flexible magnetic sheet at a hardware store; my local "Do It Best" carries some tape and rectangular pieces of it. You may have to look around for stock large enough for the stickers. You can always try a sign making place ... in fact, they may be able to make your stickers into magnetic signs for you. Otherwise you just stick the sticker to the magnetic sheet, cut carefully around the edges of the sticker, and place them on your car.

Is this legal, or do they have to be permanently attached? Anyone know?

 

There is nothing illegal about using magnets for attaching the Diamond “Access OK†stickers - as they do not specify an attachment method - but, good luck getting the magnets to stick anywhere on the front or back of a Prii. The bumpers are plastic and foam, and the hood and rear hatch are both made of aluminum.

Magnets will work for placing the stickers on the sides of the car above the wrap-around bumper.

I too am hesitant about “sticking†the stickers to the bumpers. I recently removed and replaced a street parking permit sticker. Even though I waxed the area before applying the sticker a year ago, the area underneath was discolored. Thankfully the new sticker is the same size. NO WAY am I “sticking†those “Access OK†monsters onto my Salsa Baby.

 

I'd read someplace that it is illegal, and you can get ticketted for doing it, because this would allow the same stickers to be used, potentially, on more than one vehicle without paying the proper fee.

 

I did not find anywhere - authoritative or otherwise - which states the exact method of attaching the stickers. But I did read where using said stickers on a vehicle other than the one it is registered to is a misdemeanor . . . not just a lowly infraction.
http://www.dmv.ca.gov/pubs/vctop/d11/vc21655_9.htm