I've been saving this rant for a while now:
1. Test everything at the front-end, in exquisite detail - every project sponsor understands what tooltip 0 really means. Also a great idea if you like long-running and fragile tests that require deployments, browsers, testing frameworks and the kitchen sink. Testing at different layers, and perhaps even without a browser or (in java) a servlet container is for the weak.
2. Perform a database cleanup before and after every test, whether it needs to be done or not. For the truly adventurous add something about clearing out JMS queues and stopping scheduled tasks while you are running the cleanup tasks.
3. Always use the same data for tests, and better still use the same data for different tests. That way you will have do perform anti-pattern 2 with no questions asked. If anyone does ask about random or unique data just scoff sagely.
4. For those tied to java, run each test in its own JVM. If you happen to use a DI framework with lots of XML make sure it is initialised completely for each test. If anyone mentions forkmode=once just pretend to ignore them until they go away.