From the heading of this chapter, I believe you've understood that we'll be talking about testing—lots of it. One of the most important, and often overlooked, parts of your work is testing. If you work on a project and, halfway through, realize that you have implemented something incorrectly, it could turn out to be catastrophic if you have to change the way it works.
The golden rule I follow is to use an agile method for projects and testing.
If any new feature, no matter how small, is added, I test everything to make sure I didn't override or break something else.
Giving a product that isn't functioning as it should to a client is a terrible thing to do. You need to ensure that you have tested it as much as possible.