While it might seem like common sense, many painters fail to follow these basic steps at least once.
Remove dust, dirt, and grease spots (which can ruin a smooth finish) with water, a little mild dishwashing detergent, and a cellulose sponge. Rinse walls with clean water to remove the soap residue.
Tape the trim, window, and door frames
Be sure to use painter's blue tape, which can be applied up to a week ahead. Remove tape immediately after painting, before the wall dries, so you don't peel off any paint with it.
Prime the walls.
It's a common myth that walls that have been painted many times don't need to be primed. In fact, primer helps maximize the sheen and coverage of paint and gives the finish coat a more uniform appearance.
Brush where you can't roll.
Brush on paint around trim and in the corners of walls, where your roller can't reach, with a two-inch angled brush. Extend out two to three inches from windows, doors, and moldings.
Use the W technique.
For efficiency, start in the corner of a wall and roll on a three-by-three-foot W pattern, then fill it in without lifting the roller. Continue in sections until you're finished. Paint one wall at a time.
Paint the trim.
When the walls are completely dry, tape where the trim meets the wall. Paint the moldings and the door and window frames with a two-inch angled brush.