A roller screen is essentially a metal grate that fits vertically into your paint bucket and is a handy way to get rid of excess paint on your roller. After you dip the roller into paint, run it across the surface of the screen until the paint evenly covers the roller [source: The Family Handyman].
Another useful device is a roller tray, especially if you're working on a stepladder. This sloped and ribbed tray is used to load paint onto a roller. It has a shallow well at one end for holding paint – you should have about a half-inch (1.3 centimeters) of paint there. The tray can be placed on the stepladder to prevent your climbing down every time you need more paint [source: McMillan].