Some professional painting tips are to cover floors, cabinets and counters using tape and paper or plastic sheeting, and complete any wall repairs before starting to paint. Another tip is to cut in the edges with a brush before using a roller for larger areas.
Take plenty of time to prepare before starting any paint job. Tape off edges and cover any surfaces, including the floor, to avoid getting paint anywhere it shouldn't be. Wash the walls if they are dusty. Remove any screws, pins or nails. Take off switch covers and, if possible, any other hardware such as towel racks. Use primer to coat dark walls or wood, and allow it to dry completely before applying the paint. If the wall has signs of mold, clean it with a bleach solution and use a stain-blocking primer. Otherwise, the mold can keep growing and eventually seep through the new paint.
If the wall has any damage, perform any patching, and finish work before painting. Sand patchwork well, and be sure to match any original texture as closely as possible. Allow any mud or spackling compound to dry completely before painting. Experts recommend using high quality paint. Lower quality paint doesn't cover as well and ends up taking more coats to cover the same area. It is also important to select good brushes and rollers to ensure even application and a professional-looking end result.