:

: Hold your breath for a while then blow into the can. You lungs use up the
: oxygen and you will have mostly CO2 left that you exhale into the can. CO2
: is heaver than air and sits on the varnish.

Professional painters at a recent home show, demonstrated a method which was essentially the same as Jay's.
Additionally, they punched a few holes through the bottom of the can's goove (where the lid fits in) to make it easier to remove the paint from said groove with the brush.
They then stretched a piece of saran wrap or cellophane across the can's top before putting the lid back on, and turning the whole thing upside down for storage.