Apply sunscreen 20 to 30 minutes before exposure to allow the skin to absorb it. And reapply it every two hours -- more often if you're sweating or getting wet. If you have fair skin, you might even want to begin preparing for sun exposure the night before by putting on a layer of sunscreen before bed; this will allow it to be thoroughly absorbed into the skin's outer layer. Then apply the usual coat of sunscreen the next day, about a half hour before you go outside.
The sun can also burn the sensitive skin on the lips just as easily as it fires the rest of your body. At worst, the damage can lead to skin cancer. Use a lip balm with an SPF of at least 15 and reapply often.