How should I store sunscreen to keep it effective?

Keep sunscreen in a cooler or shade, not in direct sun or a hot car. Most sunscreens lose effectiveness above 80–90°F, and a closed car can hit 120°F+ quickly, degrading chemical filters. Physical (mineral) sunscreens hold up better but still degrade. At home, store it in a dark cabinet; at the beach, keep it in your bag with a cold water bottle or in the shade.