First, obviously...find some shade. In a portrait, one on one situation, you usually have time to find somewhere to place your client so that you can shoot in an area that's lighted but not completely overwhelmed by sunlight. Make sure that you're angling your client so that no part of their face or body has a patch of sunlight on it..it should be uniformly in shade. Don't be afraid to ask your client to move to different places...I often joke that I make mine dance all around with two steps to the left, one back, 1/2 a step to the right...and stop! Perfect! :)
When you can't find shade, and you're forced to shoot in that unrelenting sunshine, you still have a few options. One is to use a hood. You can buy them at amazon.com (or most camera stores) for $20-50, specifically to fit your lenses, and keep them on hand. They will help you cut the light.
I actually don't enjoy using hoods. I keep them with me just in case, but I prefer instead to bump up my shutter speed...bump it WAY up. Shooting with a wide aperture, a variable ISO, and crazy high shutter speeds will give you the effect of a hood without cutting down too much on the light or leaving crazy patterns on your clients.
Always remember to check after a shot to ensure you don't need to adjust your settings...an overexposed shot is just as bad as an underexposed one.
I'll get into the art of backlighting, which I love to do for some sessions, in another blog. :)
I hope that helps a little. I'm a self taught photographer, who learned a lot through hours on end of trial & error, and so I'm hoping that I can save you a little time by giving advice on something I learned. :)