You already know not to leave your helmet in sunny spots like the seat of your car, right? UV light can degrade the oils used in the polycarbonate shell and EPS foam, says Bell Sports's Category Manager Sean Coffey. But, he adds, "heat in itself can be dangerous for helmets. The trunk of a car can get super hot, enough to deform the foam liner and the shell." Extreme cold can also lead to decreased performance in a crash situation, so don't store your helmet in an unheated garage in winter.
What to do: To get rid of stank, mud splatter, and those telltale white salt stains on the pads and straps, wash your helmet by hand with warm, soapy water (never use solvents and other chemicals; they can harm the plastic shell and foam). If you don't have a laundry sink, a bath or shower works fine.
Finally, says Coffey, do what you can to avoid even minor dings and drops. Instead of casually draping your lid over a brake lever, secure it to the handlebar with the strap so it doesn't fall off.