As the famous social psychologist, Kurt Lewin said, “there is nothing as practical as a good theory” and that applies to User Experience design as well. Without systematic thinking about a problem to solve, we can only be as effective as chickens pecking in the pen, finding some grain here and there (solutions that work), but nothing that carries over to other problems. Other practitioners agree with this, and distilled their experience into a few models. Jesse James Garrett’s Elements of User Experience Design is the most well-known, a classic really. For a summary, see this keynote presentation.