bumper stickers are a tried and true way of that. Right now they are just $1 at CafePress with promo code BUMPIT.
These usually for over $4 each, so stock up for the next ten cars you own. I actually have a few, but I leave them on my fridge or on my laptop. Mostly because when I'm driving I would rather the other guys on the road look at me, not my thoughts on global warming.
Click here to view the full list of bumper stickers. Be sure to add promo code BUMPIT at checkout to get the $1 offer.