I'm totally doing this from now on:
Tricks of the Trade: Painter
After opening a new can of paint, use a hammer and a slender nail to punch holes about an inch apart around the bottom of the circular groove around the can opening. The lid seals on the sides of the opening, so any paint that drips into the groove will flow back into the can. This makes it much easier to open the lid the next time (as it won't be glued shut) and reduces drips on the outside of the can when the groove overflows during painting.