Do you want physical or chemical blockers? Mineral sunscreens reflect the sun's rays naturally, but can be difficult to rub in and leave skin white. Chemical ingredients rub in more easily and absorb UV rays, but some ingredients are associated with potential health concerns and aren't recommended for babies and kids.
Do you need water- or sweat-resistance? For a daily sunscreen, comfort and wearability are among the most important factors to consider (if you won't wear the sunscreen, it can't protect you), but for sports or a day at the beach, water-resistance may trump feel and smell as a priority.
Is the sunscreen for face or body? Delicate facial skin typically requires a more sensitive formula than the rest of your body. However, these formulas can often be far more expensive, so it pays to have one formula for each.
What's your skin type? Consistency and texture is a subjective matter -- those with dry skin may be obliged to go with something thick and emollient while those with oily skin will appreciate a lightweight sunscreen with a matte finish.
Do you have sensitive skin? Some users cannot tolerate avobenzone and other chemical sunscreens. Titanium dioxide and zinc oxide are generally considered to be the most gentle sunscreen options. Keep in mind that other ingredients, such as fragrance and preservatives, may also cause a reaction. It's always a good idea to test sunscreen on a small patch of skin before applying it liberally -- especially if you have skin sensitivities.