The 5 Rules of Data Normalization are quite handy when you're creating any sort of a database.
The rules are:
# Eliminate Repeating Groups
# Eliminate Redundant Data
# Eliminate Columns Not Dependent On Key
# Isolate Independent Multiple Relationships
# Isolate Semantically Related Multiple Relationships
# Optimal Normal Form
# Domain-Key Normal Form
Quite the facinating read. Good stuff, if you're into databases at all.