Write a lot of tests - mostly integration. Unit tests have proven more harmful than helpful - unit tests are great when the api is used so often it would be painful to change it so you don't anyway. Otherwise I want to change the api and the code that uses it as requirements change. When I'm writing string or a list I'd unit test that - but mostly that is in my standard library so I'm not. Instead I'm writing code that is only used a few places and those places both will change every few years as requirements change.