Put your makeup on before you put your white shirt on. Blush, powder or foundation can fall off your brush and down your collar. If your shirt is buttoned up, drape a piece of fabric over your head while getting into your shirt. If you need to touch up your makeup at work, tuck a paper towel or tissue into the neck of the shirt first.
Keep your foundation on your face. Blend it well at the jawline, but don't blend it down your neck. Foundation, bronzer or any makeup that is on your neck can transfer to the collar.
Apply your perfume before you put on your shirt. Keep your scent behind your ears to avoid turning the collar yellow.
Use hair styling sprays or gels before you put your shirt on. Hairspray will turn yellow if accidentally sprayed into the collar.
If your hair is shoulder-length, keep it outside the collar. Oils from your hair can discolor the fabric.
Read your label to follow the manufacturer's laundering instructions. Remove any stains as soon as you can to avoid having them set in. If safe to do so, dab plain white vinegar into the stain with a clean, white cloth. Baking soda or mild soap will also work as a spot treatment. Launder your shirt as usual.