Wash the siding thoroughly, using a detergent containing mildewcide and formulated for exterior wood. A power washer will make this job easy, but follow equipment instructions carefully because you can damage wood or force water under the siding with improper use.
Repair or replace loose and cracked or rotting joints and boards. Set loose nails and screws, and remove house numerals, awnings, shutters, and other things you don't plan to prime and paint.
Prime the siding and trim with a good quality exterior primer. If you use an airless sprayer, you will get much better adhesion and penetration by rolling or brushing the primer after it's sprayed, working in sections while the primer is still wet.
Caulk gaps, cracks and nail holes, using paintable silicone caulk.
Paint the siding with two coats of 100 percent acrylic or acrylic-urethane paint, using either an airless paint sprayer, brush, or roller and brush. Follow directions for drying time between coats--this is usually a minimum of four hours.
Paint the trim last with a brush and 6 inch roller, using two coats of your chosen trim paint.