These are the schema design commandments I live by, they are kept here purely for my reference.
Rules marked with * are absolute and if not implemented will result in karma so bad you will be reincarnated as a bottom feeder.

1.) Pluralise table names.
2.) *Every table in the db will have a primary key, without exception.
3.) Be consistent with Case and separators when naming tables and columns
4.) Enforce referential integrity with foreign keys.
5.) Avoid acronyms wherever possible when naming tables and columns.
6.) Avoid using the float datatype for columns that will store monetary values.
7.) Use varchar instead of char(n) unless specifically required.

If you have any rules you swear by then please comment with your reasoning.
This is a work in progress.