There are a few tried and true rules regarding outdoor portraits. First, don't shoot in direct sunlight. There are a lot of reasons for this. For starters, the light at that time of day is very harsh, so any light on the subject will be very sharp and unflattering. So even if the sun is to the side of the subject the shadows cast across the face will be very distinct. If the sun is behind the subject the backlighting will cast a nice rim light on the hair and shoulders, but the face will be dark and there may be some flare from the light. You may be able to make this work with post processing, but it's not optimal. Finally, with the sun in the front to light up the face will usually cause the subject to squint because of the sun beaming directly into the eyes.
The second rule is to shoot outdoor portraits either in shade or during the golden hour. The golden hour is from sunrise to one hour afterward, or at the end of the day and one hour before until sunset. This time is nice because the light is very soft and casts a golden glow on the subject. Shooting in shade can pose its own problems if the background is in direct sun and much brighter than the shaded subject.
So here are my rules for shooting outdoor portraits. I make it a rule to not shoot in direct sunlight, but if I do I will make my own shade. Any translucent material that can be held over the subject to soften the sun's light will do. I have an 8' diffuser that I use for this purpose. Then I proceed to shoot as if I were in shade (explained next).
Most of the time when I shoot outdoor portraits I do one of two things. I either shoot late in the day when the sun is low on the horizon, or I shoot in shade. Living in Texas we have very long days which means most of my shoots occur at suboptimal times. This means I usually have to find a nice shady spot for shooting. There are two major things I try to remember when shooting in shade. First, be sure to get your white balance right. If you don't get this right your colors will not be right and your image will be muddy, or have a green or blue tone. Second, expose for the foreground and light your subject with speed lights.
Lighting your subjects with speed lights can be done one of two ways. First, you can light using the manual mode. This means to take an exposure with your meter and then set your lights to match that exposure. To do this you will need a light meter like the one you use in your studio to measure those lights. This is incident metering which means it only accounts for the light that is striking your subject to determine the meter reading. The second way to use speed lights is to use the "Through The Lens", or TTL metering. TTL metering means to let the camera determine how much flash is needed. So if you meter your exposure to be F-8, 1/500, at ISO of 400, then TTL will use reflective metering to determine how much flash is needed for that exposure. The problem with TTL is that it uses the entire scene to determine the amount of flash. So if the background is fairly bright the metered amount may assume there is plenty of light and send very little flash. This may result in a dark foreground. So my rule for using TTL in shade with a bright background is to expose for the foreground using your camera's reflective metering. This way the flash will try to balance the light for the foreground and will probably darken the background. This almost always works, but sometimes still leaves the background too bright. In that case I will bring the exposure down on the camera to darken the background (like the sky) and increase the flash compensation by the same amount so the foreground is lit appropriately.
Finally, I try to position the main light above and to one side of the subject, usually the short side. Then on the opposite side I will place another light to add some hair and rim light. Below are some samples of shots I have taken recently using the techniques described above.