How often should I clean the dryer's exhaust vent system, and how?

Lint should be removed from the exhaust vent at least every 2 years, or more often depending on dryer usage. You should also remove lint and debris from the exhaust hood regularly, use no more than four 90° elbows in your vent system, and replace plastic or foil vent material with 4" (102 mm) diameter heavy, rigid metal vent material. Service calls caused by improper venting are not covered by the warranty and will be paid by the customer. (Pages 5, 13)