Far more important than the sunscreen you choose is the way you apply it. An SPF 50 sunscreen applied haphazardly provides much less protection than an SPF 30 (or SPF 15) sunscreen applied conscientiously.
Apply 15 minutes before sun exposure. This is true of all sunscreens.
Use massively more sunscreen than you think you should. A rule of thumb for a person wearing shorts and a T-shirt is to use at least an ounce (visualize a full shotglass), and make sure you cover every square inch of exposed skin.
Reapply at least every two hours. True for all sunscreens, this is another guideline that many people are lax in following.
Pack enough for the trip. Though it might sound like a sunscreen sales ploy, dermatologists will tell you that two people on a four-hour hike on a sunny day should use an entire 4 fl. oz. tube. To put it another way, if you’re looking at an old tube’s expiration date, you might not be bringing enough (and you likely didn’t apply enough last time you used it).
Use a sunscreen rated SPF 15 or higher daily. This is a recommendation from the Skin Cancer Foundation. The foundation suggests SPF 30 or higher for extended outdoor activity.
Sunscreen and infants: Use only shade to protect kids under 6 months of age because their skin can easily absorb sunscreen.
Spray sunscreens: The FDA recommends against using sprays on kids because of the likelihood of inhalation and respiratory problems like asthma. In addition, sprays encourage the application of too little sunscreen. Sprays can be handy for applying over thinning hair, though a hat is still your best bet there.
Sunscreen expiration dates: A rule of thumb is that a sunscreen is good for up to three years, though the best indicator is the “use by” date on a product. Whenever possible, store sunscreen in a cool dry place because heat and humidity hasten its demise.