Make sure to test out your paint swatches in the room so that you can see what the colors look like with natural light and in the evening with the lights on in the room. Often artificial light will change the look of the paint color by casting a more yellow or blue tone to the color of paint so it is important to always test your paint swatch in the room before painting.  Wipe down the walls with a damp cloth to remove any of the dust from sanding to prevent the dust from sticking to the paint. If the walls have any grease on them use a cleaner designed to remove grease on the walls as the paint may not adhere to greasy walls. If you wish to paint over dark walls make sure to paint a coat of primer before you begin painting as this will reduce the number of coats of paint you will need to paint later. When painting near the ceiling use an angled brush as they will help prevent getting paint on the ceiling.