get yourself and some friends a pack of 5 of these stickers, designed by spacedandii.
 
these are small ~2.5inch stickers (photo has my hand for some semblence of scale) made of vinyl to last a long time!
 
I have 12 packs of 5 - I will be donating $120 to PCRF, aka all the proceeds from these go stickers go right to this charity. I'll get this money donated by the end of the month!