Best practices for clean tests

What are your best practices for writing clean and maintainable testing? Any kind of: unit, integration, component, etc

I am not writing the testing, I might adapt a tool or build a script to facilitate my testing.
Testing is a sapient activity. I use my brain to learn through experiments and explorations, in order to find quality threats to the value of the product.

Cem Kaner already wrote something about ‘good tests’ in his Test Design course.
You can access it through http://testingeducation.org/BBST/testdesign/BBSTTestDesign2011pfinal.pdf
Page 339 to 360.
Additional learning materials on http://testingeducation.org/BBST/testdesign/ :
http://testingeducation.org/BBST/testdesign/CollardUsecaseTesting.pdf
http://testingeducation.org/BBST/testdesign/Kaner_GoodTestCase.pdf