If you're painting over a new primed wall, you can safely skip the preparation step of scrubbing and sanding the surface you want to paint. But if you're painting over a previously painted surface, look for rough, peeling, or chipped areas. The best way to find flaws is to remove all the furniture from the room. If this isn't possible, cluster the furniture in one area, and cover it and the floors with drop cloths. Take down the draperies and the drapery hardware. Loosen the light fixtures; let them hang and wrap them with plastic bags. Remove the wall plates from electrical outlets and switches (if you intend to paint them the same color as the wall, do so while they're off the wall). If you find flaws, now is the time to fix them. You don't need to take a weekend or a week to tackle interior fixes. Instead, you can break it down into smaller jobs -- quick fixes that take just an hour or two each.
After fixing any flaws, wash down the surfaces to be painted with warm water and a good household detergent or wall-cleaning soap to remove soot, grease, cigarette smoke, and airborne dirt. Using a sponge just slightly less than dripping wet, go over a vertical strip of wall about 2 feet wide. Squeeze the dirty water out of the sponge into a separate pail or down the drain. Go over the wall with the squeezed-out sponge to pick up as much of the remaining dirt as possible. Squeeze out the sponge again, and rinse it in clean water. Then, sponge the same area once more to remove the last of the dirt and detergent residue. This routine sounds tedious, but it actually goes fast, and you'll end up with a wall that is clean and provides a good surface for a new coat of paint.
Don't attempt to paint over a surface that already has a glossy finish, even if it is clean. Glossy surfaces don't provide enough adhesion. And even if the paint goes on, it may not stay on. To cut the gloss on an entire wall, wash it down with a strong solution of trisodium phosphate (TSP), available at hardware or paint stores. Mix the TSP powder into hot water until no more will dissolve. Swab it on the wall, and sponge it dry. Rinse with clear water, then sponge dry again. If TSP is not available (in many communities it has been banned because of its tendency to pollute water sources), you can use a commercial deglosser, a solution that you swab on glossy surfaces before painting.
You can use deglossing solutions on woodwork, too, or you can give woodwork a light sanding with medium- or fine-grade sandpaper. Wipe off or vacuum the resulting powder before you paint. On baseboards, remove accumulations of floor wax or acrylic floor finish with a wax remover or finish remover.