Here are two tricks to wipe out the most common types of sunscreen stains.
Looks like a darkened splotch or ring–similar to when you drop salad dressing on your blouse. To treat this type of stain, take a clean, damp sponge and dab a small amount of Pine Sol on the spot. Let it sit for 15 minutes before throwing it into the wash. Just make sure to check that the stain is completely out before tossing it into the dryer, as any heat will set it permanently.
Looks like a bit of orange or pink on your clothes–similar to when you accidentally bleach colored clothing. Many sunscreens have an ingredient called avobenzone in it. When that comes into contact with water (or sweat), it creates a chemical reaction that causes discoloration. To treat these types of stains, sprinkle them with baking soda to soak up any excess oil. Let it sit for half an hour. Then, brush off any excess powder and rub the stain with clear dishwashing soap. Let that sit for another ten minutes, and then machine or hand wash as normal.