What methods and approaches do you use when developing tests to ensure their effectiveness?

To develop effective tests, I use the following methods and approaches:

• Requirements planning and analysis — I clearly understand what exactly needs to be tested.
• Coverage of main scenarios and boundary cases — to make tests as useful as possible.
• Dividing tests into levels: unit tests for logic, integration tests for component interaction, e2e tests for user scenario verification.
• Using mocks and stubs — to isolate the tested components.
• Test automation — so they can be run regularly and provide quick feedback.
• Keeping tests up-to-date — updating them when code changes.

Example: when writing a unit test for a discount calculation function, I create tests with different input data, including zero and negative values, to verify the correctness of the operation.