7 rules for taking photographs of people

1. The Crop: Think how you would like the photo to look, frame it in the viewfinder, then step back/zoom out and allow a little bit more, you can always crop it back in post. Also be aware of what isn't in the photo, cropped feet and heads can be very distracting.
2. Light/Shadow: Look at how the light falls on the subject and check for unflattering or odd shadows or highlights. For general portrait work a light at 45° from the subject gives the best mix of shadow and light.
3. The Pose: A staged stiff pose can make the difference between a good photo and a cheesy one. Try to keep things as relaxed as possible and don't make your model hold the same pose for too long without taking a break.
4. Background: Unless you are setting a scene, plain backgrounds tend to work best and don't distract from the main subject of the photo. A lamp stand out of the head does not look flattering!
5. Angle: The angle you take a shot can make a big difference in perceived proportions. Taking from below can make the subject appear taller but could also give them disproportionate feet! Try to aim at the middle of the body when taking a full standing shot; also don't be afraid to rotate your camera.
6. Focus: What is and isn't in focus is very much dependant on the field of view, be aware of what is and isn't focussed especially if you have an area of the shot that you want to draw attention too, this should be your main focus. Most auto-focus systems will focus on the centre of your shot by default, adjust if necessary.
7. TMSF: Take many, show few. One of the biggest mistakes with photography is to show people everything you have taken. Take as many shots as possible but select only a few of the best shots to make public. 2 perfect shots out of 50 taken would be considered a good session.