Next time you paint with a roller, line the tray with aluminum foil before filling it. If the tray is large, use extra-wide foil. Press the foil down so it conforms to the inside of the tray. Use your fingertips and palms — fingernails can puncture the foil. Fold the foil over the tray edges and crimp it in place. Then pour in the paint and load the roller as usual. When you're done, pour any excess paint back into the can, then peel off the foil liner and dispose of it properly.