In my experience, in the order of importance: 1. hire qualified people 2. document all the requirements and business decisions 3. rigorous design reviews 4.

In my experience, in the order of importance: 1. hire qualified people 2. document all the requirements and business decisions 3. rigorous design reviews 4. mandatory code reviews 5. implement tests for each requirement 6. have a regression testing suite 7. have a user acceptance test environment, separate from production