Toby Samples tweeted earlier an old blog post by Jeff Atwood about the virtue of minimal code, and I think everyone would be well-served by reading it and the articles it links to (the Wil Shipley post has since moved (incidentally, if you don't currently pay attention to Wil Shipley, you really, really should)).
I've found maintaining the goal of reducing the conceptual weight of my code to be the most valuable tool in my belt. One way of doing that is to separate out unrelated concerns, letting you focus only on the task at hand, but the best way is to eliminate concerns from your code entirely. It's not entirely a matter of "don't reinvent the wheel" (if your whole project is the wheel, feel free to reinvent it), but rather more of a matter of not solving problems you don't have to solve. For example, have you seen the class ExtLibUtil? It solves tons of basic little problems that most XPages run across, such as getting the viewScope. Back when I learned about that, I dropped a bunch of my existing utility functions. I'm presently doing the same with StringUtil. Those are small problems that I have no business re-solving.
I could go on, but I think the links in the first paragraph cover it best anyway. Go read them!