• Test method names should be sentences (This_method_does_this(){})
•  Test the happy path – the most common/important functionality (acceptance criteria should be executable)
• Test at the highest level that is practical
• Unit Tests should not:
• Talk to the database
• Talk to the network
• Touch the file system
• Don’t change business logic to write the code
• Depend on any other tests (can be run at any time, in any order)
• Depend on environment variables (USE MOCKS!)
• Tests should be fast (lengthy tests are doing something wrong)
• Less than 10 lines of code
• Only one or two logical asserts per test
• Don’t write tests after development is done 

Further reading:

• Introducing BDD
• A Set of Unit Testing Rules