In one sentence, describe what the following article is about:

You should wear sunscreen each time you go out during the day, even when it's overcast. Make sure you select a sunscreen with a strong enough sun protection factor (SPF) to protect you from UV rays. Go for a sunscreen of at least SPF 30. The SPF should be written somewhere on the sunscreen bottle. If you have cancer, or pre-cancer, get a sunscreen with SPF 45 or higher. Look for the words "broad spectrum" on the bottle. This assures you the sunscreen protects you from UVA rays in addition to UVB rays. Do this each time you leave your house during the daytime when the sun is out. This is especially important if you'll be exposed to the sun for more than 30 minutes. If you have trouble remembering to apply sunscreen, try leaving yourself a note on the door before going out. Keep track of how long you've been outside. You should make the effort to reapply sunscreen every 2 hours so it stays effective. If you're inside all day, and go back out before the sun sets, you should also reapply sunscreen. If you've been swimming, you may want to reapply sunscreen after getting out of the water even if two hours have not yet passed. Many people do not realize how much sunscreen you actually need for adequate protection. You need at least 45 milliliters (1.5floz) of sunscreen to cover all exposed skin on your body. This is about as much sunscreen as could fit in an average shot glass. Smooth sunscreen over rather than rubbing it. Make sure to cover any skin that will be exposed, including skin on your back. Have someone else help you apply sunscreen if there's any spot you can't reach.

Summary:
Choose a sunscreen with a safe SPF. Apply sunscreen 20 to 30 minutes before leaving your home. Reapply sunscreen every 2 hours. Apply the correct amount.