An adult needs approximately two to three tablespoons of sunscreen (30 to 45 ml) lotion to cover the whole body, and one teaspoon (5 ml) to cover the face and neck. Studies show that most people only apply a quarter or a half of the amount of sunscreen needed to get the SPF displayed on the product bottle, so apply generously!
Above all, don't forget to reapply it every 2-3 hours or after being in the water, doing physical activity or after sweating a lot and to use a minimum SPF 30 lip balm.
Precautions such as wearing sunglasses, wide-brimmed hats, and long-sleeved clothing (in summer too) are recommended by the Canadian Dermatology Association.