Get estimates from several contractors, request and contact references, search for complaints at the Better Business Bureau and check that the company is properly bonded and insured, as well as licensed in your state . Only pay a small deposit, and don't pay the entire amount until the job is finished to your complete satisfaction (both painting and cleanup).
Begin prepping your walls by removing any outlet covers or lighting fixtures (if possible). Next, inspect your walls for any defects. If you encounter cracks, dents, or holes in your walls, simply fill them in with caulk or spackling compound. There are also a number of DIY tips to fill nail holes. Once your repairs dry, smooth them down with sandpaper and wipe the walls down with a damp sponge.
Like the rest of your home’s exterior, you should start from the top down when painting exterior trim. If your window sills look particularly weather-beaten, you might want to give them a second or third coat of paint. When painting doors, paint the panels first, then the rails and stiles, and finally the edges. For more details, read up on trim painting guidelines.