It is all about the light. Welcome to a research project that could last a lifetime!

A few tips:

• The subject should be lighter (or darker) than the background but there should be good contrast.
• The primary light source should be from the side and away from the photographer. 45 ° is suggested. This is the old "over the photographer's shoulder" tip.
• If the background is bright use a "fill flash".
• "Soft" light is better than "hard" light in some instances.
• The "golden hour" can be magical.

Typical 3 light setup for portraitscredit

Examples:

Soft and hard light examples

Back Light and Fill Flash

Composing the image is all important...

All rules are made to be broken... but you better know them first and maybe break just one at a time. -example--example-

The Basics...

Move closer, fill the frame. -example-  -example- -negative example-  -negative example-

KISS, can your subject be described in a simple sentence? Does is have a strong focal point/subject? -example-   -negative example- -negative example-

Rule of Thirds can help you aim your camera. Dead center can be "deadly" -click diagram- -example-&-negative example-

-Horizon placement and straightness. -example- -negative example- -negative example-

Lines: Diagonals are dynamic, horizontals are calm and static, verticals show permanence and stability. They can lead the eye into the image. S curves lead the eye calmly through the image. -example-  -example-  -example- -example-

Verical formats and other shaped crops can play a huge role. -example- -example- -example-

Space for movement, a subject's gaze needs this too. -example- -negative example- -negative example-

Contrast color or tone. -example-  -example- -negative example-  -Color Wheel-

Don't cut off limbs (see below) -example-   -negative example-

Framing and vignettes -example- -example- -negative example-

Some other composition considerations...

8 Ways to Describe and Measure Your Photos

Posing humans for portraits - this is a whole subject in itself. -example-

Balance; with objects, light and dark tones, colors... -example & negative example- -example- -example-

Depth; is there a foreground, middle and background...do you need all of those or only 2? -example-   -negative example-

Rule of odds...this is from the design world, 3 objects are more pleasantly arranged anywhere than 4 -example- -example-

Keep important things separate from other objects. Can aid in creating the 3rd dimension in a 2D print. -comparison example-  -negative example-

The "decisive moment" - Henri Cartier-Bresson -example-   -example-  -example-

Backgrounds can make it or break it. -example-  -negative example-

Work the angles, especially with small subjects -example & negative example-

What is the least interesting  part of the image? Can you remove it? What is the least interesting now?... -example-

Work the idea, don't be satisfied with your first shot. -example-  

Using Depth of Field choices can greatly affect the final image.
-Shallow DOF- -Deep DOF-

Good web posts on photographic compositions:

Digital Camera World - 10 Rules of Composition and Why they Work: http://www.digitalcameraworld.com/2012/04/12/10-rules-of-photo-composition-and-why-they-work/

12 Top Composition Rules:http://www.ephotozine.com/article/12-top-photography-composition-rules-17158

Digital Photography School - Composition Tips: http://digital-photography-school.com/digital-photography-composition-tips

7 Photography Cliches that are still Fricken Awesome: http://www.lightstalking.com/7-photography-cliches-that-are-still-fricken-awesome/

Five Ways to Get a Real Smile From Your Portrait Subject: http://www.lightstalking.com/five-ways-to-get-a-real-smile-from-your-portrait-subject/

Angles...

Rule of Thirds...

Frame Your Subject