If you've got an hour to spare, check out Adam Savage's speech from Maker Faire 2010 in California. He talks about problem solving - specifically HIS methodology - and while listening to it, all I could think was that this was a perfect methodology for robot design.
Some of the talking points may not necessarily fit with NXT robot design or programming (such as budget issues or location - or maybe they do!)... but I think you'll get the picture. He's provides a very organized list of questions to ask yourself as well as reminders for when you're deep in a project.
The entire thing is fun to watch, especially if you're a MythBusters fan, but if you don't have the time, skip to part 3 and watch it and part 4... definitely worth the time.