When most folks think of portrait photography, they envision studio lighting, canvas backdrops, and a camera perched upon a tripod. But many photographers don't have access to lavish professional studios, and honestly, it's not necessary for dynamite portraits.
All you really need is a willing subject, a decent outdoor setting (preferably with trees), and your digital camera, and you can be on your way to creating outstanding images.
First, start with the magic rules for great outdoor portraits:

Try adding supplemental light from the flash or a reflector.

Turning on the flash outdoors is a trick that wedding photographers have been using for years. If you really want to impress your subjects, position them in the open shade (such as under a tree) with a nice background in the distance. Then turn on the fill flash and make sure you're standing within 10 feet (so the flash can reach the subject). The camera will balance the amount of light from the flash with the natural background illumination, resulting in an evenly exposed portrait. A variation on this technique is to turn off the flash and use a reflector to "bounce" the light back toward the model's face. The advantage here is that the reflected light is softer than that from a fill flash.

Learn to love high clouds and overcast days.

High clouds turn the entire sky into a giant lighting softbox, and all of nature becomes your portrait studio. The diffused lighting is perfect for outdoor portraits, and it softens skin tones. In this situation, you have the option of shooting without a flash and reflectors. If you want to brighten up your model's eyes a bit, turn on the fill flash or use a reflector. One thing to remember when shooting in this type of lighting: change your white balance to Cloudy, or the skin tones may be too "cool."

Get close.

The tighter you frame the shot, the more impact it will have. Extend your zoom lens and move your feet to create more powerful images. Once you've moved in close and have shot a series of images, get closer and shoot again.
Once you've found a setting that you like and have everything in order, you can begin to "work the scene." Start by taking a few straightforward shots. Pay close attention while you have the model turn a little to the left, then to the right. When you see a position you like, shoot a few frames. (Don't get too carried away with this "working the angles thing," or your subjects will hate you. You're not a swimsuit photographer on a Sports Illustrated location shoot. The point is, don't be afraid to experiment with different camera positions. Just do it quickly.)
Next, move in closer and work a few more angles. Raise the camera and have the model look upward; lower the camera and have the subject look away. Be sure to take lots of shots while experimenting with angles, because once you've finished shooting and have a chance to review the images on your computer screen, you'll end up discarding many images that may have looked great on the camera's LCD monitor. When they're enlarged, you'll see bothersome imperfections you didn't notice before.
When shooting portraits, communicate with your subjects and try to put them at ease. Nobody likes the silent treatment from the photographer. It makes your subjects feel like you're unhappy with how the shoot is going.
Here are a few other things to avoid when shooting outdoor portraits:

Avoid harsh side lighting on faces.

Light coming in from the side accentuates texture. That's the last thing most models want to see in their shots, because more texture equates to increased visibility of skin aging or imperfections. Use a fill flash or reflector to minimize texture, and avoid side lighting unless for special effect.

Don't show frustration.

Never, ever make subjects feel it's their fault that the shoot isn't going well. They're already putting their self-confidence on the line by letting you take their picture. Don't make them regret that decision. When shoots go well, credit goes to the models. When shoots go badly, it's the photographer's fault. Keep your ego in check so theirs can stay intact.

Avoid skimping on time or the number of frames you shoot.

Your images may look good on that little 2" LCD monitor, but when you blow them up on the computer screen, you're going to see lots of things you don't like. Take many shots of each pose, and if you're lucky you'll end up with a few you really like.

Don't torture models by making them look into the sun.

Yes, you were told for years to shoot with the sun to your back—but the photographer, not the model, devised that rule. Blasting your subjects' retinas with direct sunlight is only going to make them squint, sweat, and maybe swear. Be kind to your models, and they'll reward you with great shots.

Avoid busy backgrounds.

Bright colors, linear patterns, and chaotic landscape elements will detract from your compositions. Look for continuous tones without the hum of distracting elements.
Now that the basics are covered, here are a couple of super Pro Tips. Make sure you have some good, solid shots recorded on your memory card before you start experimenting with these techniques. But once you do, try these suggestions:

Soft background portraits

These are simply lovely. A soft, slightly out-of-focus background keeps the viewer's eye on the model and gives your shots a real professional look.

 Rim lighting for portraits

When you place the sun behind the model, often you get highlights along the hair. Certain hair-styles really accentuate this effect. Remember to use fill flash for this setup, or your model's face will be underexposed.