Not only should you not be the sole tester of your own software, but you should also not be responsible for generating the test data, test scenarios, or test plans for your software. If you are, you may make the same mistakes in test generation that you made in software creation. For example, if you made a false assumption about the range of legal inputs when engineering the software, you would likely make the same assumption when generating test plans.
If you are a programmer and/or designer and your manager has asked you to write your test plans, I recommend you switch the test plan generation responsibility with a fellow programmer and/or designer. If you are a member of a requirements engineering team, with responsibility for system test generation as well, I recommend that members of your team subdivide the responsibilities so that no individual generates tests for requirements that she or he wrote.
