Make sure attic vents function properly. They should not be blocked with storage items or insulation from inside your attic, and should be clear of branches, leaf litter, and cobwebs from the outside. Clear and clean vents mean that air circulates properly through your attic and keeps moisture and mold problems at bay.
Check to make sure your clean vents are covered with wire mesh to keep rodents out of your attic. Holes in the vents should be no larger than a dime.
Excellent information about roof ventilation can be found here.
