A Good Testers need to follow ten basic principles:
1. Always think from the end-user/ customer point of view
2. Understand the requirement clearly and technically
3. Use your logical sense to the best of ability
4. Do not be a friend to the developer.
5. Look at the test cases from all angles- coverages, acceptance, co-relations, impact, dependencies etc
6. Learn to write good and comprehensive test cases
7. Learn to report defects appropriately- with test steps, sequences, patterns, screenshots of the defects etc
8. Followup on the defects till they are closed appropriately
9. Report issues to the right persons for immediate resolution
10. Remember you are the gatekeepers for the quality of the product to be delivered
Simple and realistic ten steps would not only make you a good tester, but would also make the products delivered consistent, reliable, competent and overall makes the application user friendly and meaningful.