You must prototype it in code

Jeff Atwood in Good Programmers Get Off Their Butts:

I am not proposing a code-like-hell methodology. I am merely observing that, in my experience, coding without planning is just as futile as coding with too much planning. Software development is a wicked problem; you should never make planning decisions without some kind of code prototype to ensure that you're making informed decisions. If you plan too far ahead of the code, I guarantee you are doing work that will be thrown away or altered until it is unrecognizable.

The most destructive symptom of over-planning is the wrongheaded idea that being a Software Architect(tm) means drawing a lot of UML diagrams and handing them off to a group of developers in Bangalore. UML is great if you don't want to do any work; you just draw pictures of what it would look like if work was actually done. This is not only borderline laziness, it's also a recipe for disaster. You can't architect a real world application on a whiteboard. You must prototype it in code to gather data on the performance and design implications of the choices you are making.