Summarize:

Between 10 and 3, the sun's rays are at their most powerful and most direct. You'll receive large amounts of UV light if you're outdoors during this time, which will darken existing sunspots and can create new ones. So, postpone running errands and jogging outdoors until after 3:00, or do your outdoor activities early in the morning. If you have sunspots, never use a tanning booth. Tanning booths pose health risks to your skin in general, and using them will darken your sunspots. Exposure to the sun causes the majority of sunspots. To protect your skin from further sun damage, slather a high-SPF sunscreen onto any exposed areas of skin (e.g., hands, neck, face, and arms) if you're going to be outside for longer than 15 minutes. Using sunblock will also keep any existing sunspots from darkening. If your skin has already been damaged by the sun, using sunblock to protect yourself from the sun can also allow the skin to heal itself naturally. Even if you're wearing sunscreen, it's smart to further protect your skin by covering it with loose clothing that will block the sun. Wear light-colored clothing with a tight weave to deflect the sun's damaging UV rays. Keeping direct sunlight off of your skin will help existing sunspots fade over time and prevent new ones from emerging. You could also carry a light umbrella with you to protect yourself from sunlight.
Stay out of direct sunlight between 10 am and 3 pm. Apply sunblock with an SPF of at least 15 if you must go out. Wear a hat and protective clothing when you go outdoors.