A checklist from Agile in a Flash, for basic TDD-/BDD-style test writing:

• Test that you can instantiate the class
• Tests that you can call the method at all.
• Pick the easiest bit of functionality.
• Pick the most interesting bit of functionality.
• Write the function name and assertion first
• Rename something
• Refactor the existing tests
• Switch partners
• Read the code for obvious flaws
• Rerun the autotest
• Reduce your scope