When was the last time you went outside for a normal day of activities and actually applied sunscreen first? Have you ever wondered how much sunscreen to use or which brand is better?

Dermatologists agree that sunscreen is a great tool for skin cancer prevention — but only if it's used correctly. Here's what you need to know about getting the best sun protection this summer.

The American Academy of Dermatology recently published a study in the Journal of the American Academy of Dermatology that found many people are making mistakes when applying sunscreen. Researchers set up several free sunscreen dispensers at the Minnesota State Fair and watched as 2,187 people used them over the course of 93 hours.

Only 33 percent of people applied sunscreen to all exposed skin most only applied it to their arms and faces. Just 38 percent of fair-goers were wearing sun-protective clothing, hats or sunglasses. Researchers also found that use of the free sunscreen stations decreased drastically on cloudy days, even though 80 percent of the sun's harmful rays still shine through the clouds.

"These results highlight some of the ways people use sunscreen incorrectly," Ingrid Polcari, M.D., FAAD, an assistant professor in the department of dermatology at the University of Minnesota Medial School in Minneapolis said in a press release. "To get the best possible sun protection, it's important to wear protective clothing, such as long-sleeved shirts and pants, and to apply sunscreen to all exposed skin, not just your face and arms."

The American Academy of Dermatology recommends an SPF of at least 30 and says to look for "broad spectrum" on the label. SPF, which stands for sun protection factor, measures how well a sunscreen guards against ultraviolet B (UVB) rays. These harmful rays are the main cause of those painful sunburns and a huge contributor to skin cancer. Keep in mind that the intensity of UVB rays varies depending on the time of day and your location.

All sunscreens should be reapplied every two hours you're in the sun. Doctors say to apply about one ounce of sunscreen (enough to fill a shot glass) per person at each application.

Researchers from the Mayo Clinic recently published a paper reporting increased diagnoses of two types of skin cancer in recent years. The study, published in Mayo Clinic Proceedings, compared diagnoses of basal cell carcinoma and squamous cell carcinoma both nonmelanoma forms of skin cancer between 2000 and 2010 to diagnoses in prior years.

Researchers reported that during that time frame, squamous cell carcinoma increased 263 percent, and basal cell carcinomas increased 145 percent. Those alarming results make choosing the right sunscreen even more important.

If you think all sunscreens are created equal, you're wrong. Consumer Reports has found that sunscreens advertising high SPF ratings aren't always a great measure of how much sun protection you'll actually get.

Of the more than 60 lotions, sprays, sticks and lip balms tested in 2017, 23 tested at less than half their labeled SPF ratings. That means sunscreen labeled as SPF 50 may only deliver an SPF 25 level of sun protection.

The organization's top pick is the La Roche Posay Anthelios Melt-In Sunscreen Milk SPF 60, which retails for $36. The group's second pick is the Trader Joe’s brand Spray SPF 50+, which costs $6. Finally, Consumer Report's third choice is the Equate Sport Lotion SPF 50, which retails for $5. Consumer Reports magazine will release the full report and rankings in its July issue.

The bottom line is that sunscreen only works if you use it, so be sure to lather up regularly and go outside and have some fun!