I'm pretty sure that, by now, you've noticed I keep telling you: testing, testing, and more testing. So, I guess there is no reason to say it again here. Maybe, just once more—when you develop, make sure you do testing, testing, and some more testing!
Your basic test plan should include your deadlines, budget, and assessments of what would be a realistic goal. You should keep track of your changes via documentation and be sure that you have some sort of source control in your project.
In the next chapter, we will discuss the design concepts that can be followed to help improve a site's performance.