For dryers, a vacuum cleaner can be used to clean the dryer pocket where the lint trap sits. You will need to unplug your dryer and move it away from the wall, and remove the ductwork from the back of the dryer. Use the vacuum along with a dryer brush from a kit to clean the ductwork. Clean the back of the dryer, the floor where the dryer sits, and the wall.
For air vents, remove vent covers and use the vacuum hose and attachments to clean the ductwork as far as you can reach. Use a dry rag to loosen stuck-on dust and debris. Never use anything moist to clean air ducts. Repeat this procedure for all vents in the house. Most of the dust and allergens will be fairly close to the vent openings, so cleaning as far as you can reach will greatly improve the air quality.
A dryer cleaning kit can be purchased at your local hardware store for around $20 (as of 2010). The kit includes dryer brushes, flexible rods, and vacuum adapters. When using the rods from a kit, you will need a drill. You will connect the rods to the brush provided in the kit, and attach the assembly to a drill. The assembly will scour the inside of the ductwork when the drill is on.
Once you are satisfied that your air ducts are clean, you can place filters in the vent covers. These filters are sold at your local hardware store, and come in sizes to fit nearly all vents. The filters will keep dust and allergens from reentering the house. Change the filters regularly, because dust will collect on the outside of the filter when vents are not in use.