Rule of Thirds
The rule of thirds can assist in image composition by creating better image balance. Imagine a theoretical grid of lines (both horizontal and vertical) dividing the image into thirds. By placing your subject on the intersection of these lines you can achieve a more compelling compositon that creates a natural image balance. Some cameras can generate an image overlay that give actual reference lines in your viewfinder.
Consider the background objects that may exist in your frame. Are there any elements that may be distracting to your viewer? If so you might consider moving your subject to another location. Simplicity is generally a good rule unless the objects in the frame contribute to what your subject is talking about.
Avoid Placing Subjects in front of Windows
Cameras can do some amazing things but still aren't as advanced as our eyes. If a shot is exposed for a bright background, the subject may end up being too dark. Exposing for the subject in the example below would result in an excessively bright background, or blown out background. Try placing your subject in a space that has a similar brightness, or is darker than the light on their face.