Prevention is always the best cure to something. But it's not always possible. Sometimes you just have to minimize what is inevitable. For this kind of prevention, I recommend using a sunscreen with a SPF (sun protection factor) of 15 or more on all skin exposed to the sun. Personally I go with 30 or higher. Apply sunscreen before you head out into the sun. Repeat it a few more times during the day or depending on how long you are out. You can usually feel your skin drying out and the lotion disappearing. Remember to use a lip balm with sunscreen as well.
Remember that the sun is the strongest during the middle four hours of the day, 10am-2pm. That is when protection is most needed.
The sun is also stronger the higher you are up in altitude. I used to live in Florida at sea level and now I live at 7,000 feet. The impact of the sun was probably the most surprising difference for me. It feels hotter on the skin even when in cooler air. You burn faster.
You can buy clothing with an SPF factor. It costs quite a bit more and it is available at the usual outdoor stores. They breath and keep out the sun at the same time. They may also have more ventilation or options to zip a section off or roll it up. But it's easy to just deal with what you already may have. Hats should have a wide brim or a neck flap for best protection. Where lighter color clothing. White reflects the sun more than black which absorbs the heat and makes you feel like you are overheating faster. I just shake my head at the goth kids all dressed in black walking around desert towns in 100 plus degree heat. The clothing should be tightly woven as well. Wearing something loosely knitted with holes just lets the sun through. And remember that umbrellas are good for sun as well as rain.
Reflections can burn. Sand, snow, water all can cause burns from below. Use sunscreen on your neck, chin and areas that you think are covered by your hat.
Stay cool. Stay safe. Get out there.