A good portion of all the photos taken in these United States -- perhaps as many as 75 percent -- consists of portraits. Mostly, they're informal shots of friends, neighbors and members of the family. In theory, that's good. Few subjects are intrinsically quite so interesting as people -- their physical appearances, expressions, actions.

In reality, however, a startlingly high number of those portraits are deadly boring. You know the kind: Momma and junior flanking the sis, and all three grinning awkwardly into the camera as dad snaps the shutter.

There are ways of changing all that -- ways to add zest and life to your portraits. And it begins with you.

First, it's time you realized that you have the power to shape the finished photographs you take. Even when dealing with other people, you -- the picture-taker -- have the final say over who poses how, and when to snap -- or not to snap -- the shutter.

For openers, start working to overcome your shyness about asking people (even strangers) to pose the way you want them. Think of yourself as a director. It's up to you to stage the production just the way that's best.

If you approach you subjects firmly, yet nicely suggesting that if they do this the photo will be better, you're practically guaranteed of their cooperation. Everyone wants to look good when the prints come back from processing.

Some professional portrait photographers prefer grouping two or three people in a scene rather than shooting individuals one at a time. The reason is that, with two people posing together, there's less camera-consciousness and greater opportunity for natural-looking, spontaneous shots.

Take advantage of a multiple-subject situation: Instead of lining up two or three people firing-squad style, have them face three-quarters to the camera and interrelate. Give them some props -- something to do with their hands. Have one show the others something he's found. Have someone explaining something to the others. The possibilities are endless.

While all this interrelating is going on, you can forget your own self-consciousness and concentrate on moving around for just the right angle. Check our the composition. Move forward or back so as to get everyone into the scene. If one person is obscured by another, go up and move that person to one side or forward, then melt back, again, to your "ready-aim-fire" position.

The key to natural-looking "unposed" posed photos is to have everyone busy doing something besides primping for the camera. Sometimes a straightforward, classic-style portrait is nice, but nine times out of ten, a more natural-looking shot will be a more interesting photo.

To enhance the natural effect of your portraits, take advantage of natural light whenever possible. Pose your subjects outside, or inside near an open door or window.

Beware of the degree of light falloff, though, so that the person farthest from the light source doesn't end up hidden in the shade. If that seems to be happening, use electronic flash as a "fill" light to pop that at person back out from the darkness.

There are many occasions when you can use flash so that it nearly resembles natural light. Two of the best ways are to diffuse it through a screen or bounce it off a ceiling or nearby wall.

And always shoot your flash off-camera, either handheld (which can be awkward) or mounted off to one side on a pistol grip -- an accessory you should be able to find at your photo dealer. TIPS THAT CAME WITH TIME -- It's not especially hard to learn a few money-, time- or energy-saving tricks here and there. Not if you've been around long enough and have kept at least one ear to the wind.

Here, then, are a few things I've learned that may help make your life as a photographer just a little easier:

If you have an adjustable camera and have difficulty focusing in certain types of light, set the lens aperture (opening) at f/8 or smaller (f/11, f/16. f/22). The smaller the lens aperture, the greater the depth of field, or area of acceptable sharpness. Thus, the greater the tolerance for error in focusing without getting fuzzy images.

The larger lens apertures (f/5.6, f/4, f/2.8) offer progressively shallower depth-of-field, calling for more precise focusing for sharp images.

When you want to change 35-mm film cassettes mid-roll, you can do so without wasting the remaining unexposed frames in the camera. Make a notation of how many frames have been shot (say, 12 frames on a 36-exposure roll) and begin rewinding the film as you do normally. However, do the rewinding very slowly and pay attentionn to the sounds and tension in the camera.

As the film rewinds into the cassette, the tension will increase. Finally, you'll hear a soft "snap" as the tongue of the film's leader pulls free from the film-advance spool. You'll also notice a lack of tension. Stop rewinding immediately and open up the camera.

You should find the 35-mm cassette with three of four inches of leader sticking out -- just enough to allow you to reload that cassete sometime in the future. With a grease pencil or crayon, write "Shot through #12" on the cassette.

When you're ready to shoot the remaining frames on the partially exposed roll, load the film in the camera as usual. Then, with the lens cap securely covering the lens (and thus blocking out all light), cock and fire the camera until the exposure counter registers 13. That means the first 12 frames of exposed film are safely advanced past the shutter, and the remaining unexposed film is left to be shot.

With auto-exposure cameras, set the camera on manual while shooting with the lens cap on, then turn back to auto for the remainder of the roll.

Knowing this little trick will enable you to shoot 12 frames of color slide film followed by a roll of black-and-white followed by the remaining 24 frames of color slides -- or any combination you like -- without wasting a single frame.

When shooting color slide film in an adjustable camera, many photographers find that slightly underexposing the film results in greater saturation and more vivid hues.

To underexpose, either set the camera's exposure compensation control to -1/2 pr -1 (which will automatically underexpose all frames at either one-half or one f/stop) or double the film's recommended ASA and set the ASA dial for that figure (exposing 200 ASA film at 400 ASA and so on). Examine the processed roll of film and decide if even more underexposure is necessary in the future.

Whenever shooting closeups of people or animals, focus on the eyes. When the eyes are sharply focused, the entire shot seems sharp. When the eyes are fuzzy, the entire shot seems blurred.

For more natural-looking electronic flash shots, use the flash off-camera. Hold the unit to one side of the camera while shooting of -- easier -- mount both camera and flash side-by-side on an accessory hand grip. In either case, connect the flash to the camera via an external PC flash cord, available at most photo dealers.