There's a design principle I neglected to mention in my initial list but which certainly merits attention. That principle is this: whenever possible, don't repeat yourself (DRY). Put another way, do things one time, in one place rather than having the same or similar code scattered throughout your code base.
There are two main reasons to follow the DRY principle. The first is that it makes change easier. The second is that it helps substantially when it comes time for maintenance.
I was once told by an instructor in a design patterns class that the Y2K problem wasn't caused by using 2 digits to represent 4. Instead, it was caused by doing so all over the place. While the reality of the situation is a little more complicated than that, the principle is true. If the date handling code had been all in one place, a single change would have fixed the whole codebase rather than having to pull tons of Cobol coders out of retirement to fix all the business applications.
When I first started working on DVD, I inherited several applications that had been written to test display cards. They had started life as one large application which switched behavior based on a command-line switch to test various aspects of DirectDraw that DVD decoders relied upon. Some enterprising young coder had decided we should have separate executables for each of these tests so he made 6 copies of the code and then modified the command-line parser to hard-code the test to be run. The difficulty here is that we now had 6 copies of the code. Every time we found a bug in one application, we would have to go make the same fix in the other 5. It wasn't uncommon for a bug to be fixed in only 4 places.
Shalloway's Law: “When N things need to change and N>1, Shalloway will find at most N-1 of these things.”
This principle applies to everything you write, not just to copying entire applications. When you find yourself writing the same code (or substantially the same code) in 2 or more places, it is time to refactor. Extract a method and put the duplicated code in that method. When the code is used by more than one application, extract the code into a function call that you put into a shared library. This way, whenever you want to change something, a change in one place will enhance all callers. Also, when something is broken, the fix will automatically affect all callers.
Note that this is a principle and not a law. There are times when substantially similar code is just different enough that it needs to be duplicated. Consider the alternatives first though. Can templates solve the problem? Would a Template Method work? If the answer is no to everything, then duplicate the code. You'll pay the price in increased maintenance, but at least you'll be aware of what you are getting yourself into. It might not be a bad idea to put a comment in the code to let future maintainers know that there's similar code elsewhere that they should fix.