Composition is as important to portrait photography as it is to any other genre. That's a fact that needs to be remembered if you're hoping to be successful at portraiture, particularly outside the studio. Shooting in an uncontrolled environment presents a number of challenges and it's quite easy to overlook small composition details when you're dealing with light issues, crowds, and the like.
(Success Tip #1: Learn to take better portraits with this deck of cards!)
One of the most often overlooked elements of the overall composition in these situations is the background. We focus on the details of our subjects' faces, their clothing, framing and such, and simply don't notice anything behind them. This is one of the most common composition problems in lifestyle portraits, wedding shots, candid portraits and other photos by novice photographers and can be very difficult to correct in post processing.
Obviously, the most effective way to avoid these problems is to find an environment that provides a background that compliments your subject. Unfortunately, this isn't always going to happen when you're shooting on location. You may find yourself needing to get a little inventive.
One way to overcome the problem of a cluttered background is to open up and shorten your depth of field to blur the background elements. This can be very effective in situations that allow you to create some distance between your subject and the background. You may find this easy to accomplish in outdoor settings like parks. Remember that the color of the background will still be evident.
A different camera angle may help eliminate distractions in the background, too. Don't hesitate to change your point of view. A word of caution here: keep in mind that extreme angles can alter the facial features and body shape of your subjects. Remember, too, to compensate for a sky or similar bright background, to avoid underexposing your subject.
(Success Tip #2: Take portraits of people having fun and sell them easily!)
When all else fails, you may need to create your own background. Look for something you can bring into the shot that suits the subject and perhaps highlights his or her personality. If you move in tight, the background doesn't need to be too large.
Here's a great video from Bryan Peterson that demonstrates the last option very well. Take a look and see the great results he gets with an assistant and a simple, painted board.