Brake fluid works well but here is an interesting product -
Its called Modelstrip and i believe its made in the UK. I don't know anywhere online u can buy this, i got it from a local model shop. To use it u basically plaster the part you want to strip with the paste and leave overnight in a bag so it doesn't dry out. You then just scrub it off under running water and the paint just washes away. It works best on enamels and acrylics, I'm not sure about auto laquers coz it worked once on one model but not on another. The model it did work on had only recently been painted while the other was a couple of years old, but brake fluid soon shifted it!