Tip 10 stressed the importance of commenting your code. It's also important to do external documentation. Far too few developers write formal technical specifications for their ASP applications. This is needed, especially as the size of the application grows. Picking through source code, even well-documented source code, is more cumbersome than reading through a well written technical specification document. Also, keeping external documents allows those who are not code savvy to understand what your program is doing on a technical level.
My real world experience was mixed. Some projects I've been on were well documented with both functional and technical specification documents. Others had absolutely no documentation and, not surprisingly, no foresight. The projects that fell under this latter category often were shoddy products, difficult to maintain and update.