If you don't have automated tests, and start writing them as early as you can, you're not going to have a good time. I hate working at shops or on jobs where there is zero unit testing. You have to have tests to safely refactor things. If you miss something, you add a test. If you get a false failure; figure out what was wrong with the test.

Retesting should be as simple as running the test scripts. If it's not automated, it's not tested correctly.