One of my favorite techniques for outdoor portraiture is using backlighting to produce highlights on the subject’s hair and, depending on the scene, maybe blow out the background to create a pseudo high-key effect.
Want to try it? The next time you photograph somebody outdoors, place them in the kind of position you normally use with the sun shining onto their face but instead of making a photograph turn her around. Right away the subject is more relaxed because the sun is behind her and she won’t have to squint!
Next turn on your flash and it doesn’t matter whether it’s built-in or a shoe mount speedlight. Be sure to use flash, otherwise she‘ll appear as a silhouette in the portrait. In order to get a final image you can live with, you may have to open the aperture a stop or two over the metered exposure (or just use exposure compensation) just as you might with any other backlit subject.
Tip: If you plan to shoot full length portraits instead of close-ups, a more powerful speedlight will be more effective for fill light than the weaker pop-up flashes found on entry-level digital SLRs.