Why? To avoid potential fire hazards and having to do costly repairs to your appliances.
How? It goes without saying that you should clean your lint screen after every load of laundry. Keeping the filter clear will shorten the drying cycle, save you money, and prevent any potential fire hazards. It is also a good practice to regularly, but at least twice a year, remove the lint that accumulates at the bottom of the housing that holds your lint screen. You can generally use a vacuum cleaner's flat attachment to reach down in the housing to suck out anything that has accumulated in there. Finally, you should be cleaning out your dryer's exhaust duct using a dryer duct cleaning brush (it's a stiff bristled circular brush with a flexible handle that you can buy at most hardware stores) at least twice a year. The exhaust duct vents your dryers exhaust to the outside of your house and can become a fire hazard if not kept clear. Consider doing this more often (say monthly) if you do heavy amounts of laundry on a consistent basis. Many service providers sell regular service plans to handle this for you, so consider signing up for one.