Backlighting, or rim lighting, is critical for a professional and flattering shot. In fact, when outdoors, it is often desirable for the backlight to be much hotter than the key. This is almost impossible to do, however, if you use the sun as your key. So, what is the best approach?
Most DP's will use a combination of diffusion and bounce in order to get the best shot possible. First, face the subject with their back to the sun (offset a little is actually a good idea for eliminating lens flares and creating less symmetrical shadows). Second, put a diffusion screen between the sun and the subject to knock down the overall light level and soften shadows a bit. Third, use a bounce (reflector) in front of the subject to act as a key light. In my experience, a white bounce usually works best. Silver is usually too harsh and causes the subject to squint (as they would if they were facing the sun). Now, sometimes you will not be able to generate enough light (depending on the look you are going for) just using a bounce and it is necessary to use an HMI in place of the bounce. The picture above is from the Elk Run Productions website and illustrates this. They have a number of other examples of lighting setups at that sight that you may want to look at as well.
