It's silly but at home, I always kept my coins in the top of something, like an Altoids tin or the top to one of those Barrel of Monkeys games. Once I bought my Prius I just picked up my little Barrel of Monkeys top filled with coins, and put it in the armwrest - it fits snugly in there, it's just the right fit to avoid sliding around. Holds a ton of coins there too, and if I need the coins to leave the car - picks right up and comes with me. Of course it looks funny with me carrying around a bright green top to a child's game, but no one needs to know that's where it came from.

Prius 2006, bought September 14: