When you dip your paintbrush, you usually wipe the excess against the side of the can. Before you know it, paint is dripping down the side of the can. The little groove around the rim gets so full of paint that it splatters everywhere when you hammer the lid back on.
Avoiding the mess is easy. Just wrap one or two rubber bands around the can from top to bottom, going across the middle of the can opening.
When you fill your paint brush, just tap it against the rubber bands and the excess paint will fall back into the can.