I'm no interior designer, but I have developed 5 rules to help me decorate my house:

1. Symmetry is good
   
2. View Ways (e.g. long hallways with a picture hanging at the end) are good
   
3. Straight lines are good (especially when it comes to the top of hung pictures)
   
4. Shouting distance (between furniture) is bad
   
5. Clutter is bad
   

Developed based on Laurie Ward's Use What You Have Decorating; Sarah Susanka's Not So Big House books; and a dash of FlyLady.