What KmH said.
Also, things I heard about this:
- As always with portrait, the best option is to use a prime lens, or a high quality zoom lens, so you can use wide apertures like f/2.8 and get a shallow depth of field. Remember that in broad daylight this means that you get extremely fast shutter speeds even at ISO 100 (or whatever the optimal ISO of your camera is), so better bring a grayfilter, too.
- Advised distance for portrait is something close to about 5 meters. This is the distance which makes people look best in pictures. More distance has a tendency to make them look flat and less distance starts to distort their faces.
- Put the model between the sun and the camera. This way the sun will put a nice framing of light around the model.
- After that, try to get SOME light into the model again, especially into the face and the eyes. Not too much or your model will get uncomfortable, will get squinty, watery eyes etc.
- The most powerful option is getting a helper and various reflective surfaces.
- The second option would be using a fill flash. You might want to use a grayfilter for this, in order to be able to use slow shutter speeds below your flash x-sync even in broad daylight. Or you have an expensive camera that has the option for fast sync; but for example my D5100 doesnt allow this and you need more powerful flashes for this one too.
- Or, much easier, get into the shade. Again you want some light into the eyes of your model.
- Wait for clouds. A cloudy sky is almost ideal for portrait, except it lacks drama. Light is just equal quality from all sides and that can be boring, too. So again reflectors and/or fill flash can help.
- There is also the possibility to use natural reflectors. There might be something like white rock around.