Get a good angle on the scene
Stay aware of the sun's position in the sky, and try to put yourself between it and your subject. Sometimes that will mean walking around and shooting your subject from a different angle. It's easy to forget this bit of common sense when you're on vacation and you approach a historic attraction — your first instinct will be to shoot it as you approach and not consider other angles. Restrain yourself, though, and take a few minutes to look for better vantage points.
If you can't get a good angle, and you can't move the subject into the shade, you might be able to block the light. In my backpack I sometimes carry a flexible reflector, such as a Photoflex LiteDisc, sold in most camera shops; Amazon.com
sells them for about US$15 to US$100, depending on the size. The LiteDisc unfurls in seconds, and you can use it to shade people in front of a tourist attraction, eliminating the squinting you often get from subjects who are facing the sun.