The nice thing about shooting portraits outdoors on bright days is that there is plenty of light, but the downside is that bright sunlight often causes a lot of contrast, as well as deep shadows under the eyebrows, nose and lips. It seems kind of ironic, but one of the things that you can do to improve almost all outdoor portraits--particularly on bright, contrasty days--is to turn on the built-in flash. By turning on the flash, you open up these shadows and greatly reduce the contrast. It's important though to try and strike a natural balance between daylight and flash. If the flash is equal to the daylight it looks false and if it's more powerful than the daylight (often a problem on cloudy days) you'll get an over-lighted look.
Some cameras do a great job of balancing the flash any time that you turn it on in daylight, while others have a "flash fill" mode and in that mode the camera will automatically create a realistic balance between daylight and flash. Most DSLR cameras (like the Nikon D90 that I used to shoot this photo) also have a flash-compensation control that enables you to set the flash so that it's output is less powerful than the existing daylight. (Look in your camera's manual for info on the flash-compensation feature.) Knowing how much compensation to use is just a matter of making a quick test shot. I often take one shot with the flash compensation in the neutral position and then adjust from there--usually turning the flash down by a full stop which seems to create the most pleasing balance.
You'll know the best balance when you see it, it will just look and feel natural. Also, since you'll be using flash to light the face(s) you can often do some interesting things with light direction. In this shot of my friend Marcella and her son Hart, I positioned her so that the sun was more or less behind them and that provided a nice rim light on their hair. The flash then provided a nice even light on the face, but left the glowing highlights in the hair--it's a nice look and easy to create.
Funny, but probably the single biggest piece of advice that I give to people shooting pictures of people outdoors is to turn on the flash. And I can't remember the last time that I shot an outdoor portrait without flash, even if I had it dialed down a few stops so that it was just putting a highlight in the eyes. Try turning on the flash next time you're shooting people outdoors and I'm sure that you'll find yourself using it all the time.