Please, feel it as recommends, not as commands:
- Do precise analysis of the project. Devil is in details.
- Adopt some visualisation of db schemas and use it strictly - I recommend crow's foot notation on physical data model. http://en.wikipedia.org/wiki/Entity%E2%80%93relationship_model
- The best user friendly developing tool is pencil and paper.
- Designing tables, be very sure of cardinality/relationship and do not implement too generalized schemas (if the project requires just 1 to many relation between tables A and B, implement just this)
- Do not implement application logic onto database schema and/or stored procedures. This layer should just keep data consistent, regardless on the logic.