When shooting outdoors it's important to avoid the direct midday sun as this produces very harsh shadows. Look for some light shade such as an overhanging tree or covered seating area, where the sunlight is softer and more flattering. Alternatively, shoot in the morning or early evening when the sun isn't as strong.
If you're shooting indoors, try to position your subject near a large window so that you can make the most of any available natural light. Depending on your budget and the equipment you have available you can compliment this with some artificial lighting if necessary.
Look for a location which has some sort of shelter nearby, such as a bridge, bandstand, or cafe. These can be life-savers during a quick shower, helping you keep yourself, your equipment, and your subject dry - particularly important if they're paying!
If the weather gets really bad you may even be able to move your whole photoshoot under cover. With open-sided shelter you can often recompose to keep a natural background behind your subject, and as long as the lighting is good enough nobody will ever tell you weren't fully outdoors.
Finding a quiet, secluded location is not as difficult as it might seem. If you must shoot in a city, get off the beaten track - by moving just a few hundred yards away you can usually find a spot where you and your subject can set up undisturbed.
Better still, avoid cities altogether and head for remote beaches, grassy fields, and woodland. These all provide great backdrops to a portrait photo, and are often completely deserted, giving you free-reign to move around and experiment with different poses and angles.
Remember that every subject is a unique individual, with their own personality. This is what makes them so interesting, and it's something you should try to capture in every portrait you take. Choosing a suitable location is an important part of this.
Take the time to get to know your subject. Find out about their hobbies and favourite places and incorporate them into your photography. If they're an avid horse rider, shoot them at their stables; if they love to surf, go to their local beach.
By using a location that means something to your subject, you'll get much more personal, meaningful photos. As an added bonus, they're likely to feel more relaxed, helping you capture more natural-looking shots.