Sunscreen: Never underestimate the power of the Thai sun. Summer or winter overexposure will burn you badly and do irreparable damage to your skin. Between the hours of 10am and 3pm UV rays (the ultraviolet rays that give skin damage or worse) are at their strongest.
Wear a hat, wear wrap type sunglasses with EPF10 (eye protection factor), wear a strong SPF factor water-resistant sunscreen. Reduce your exposure to the sun to short bursts. Permanent skin damage starts to occur after 15 minutes. The tropical sun is not to be trifled with, do not underestimate it.