Any advice or lessons learned for those re-organizing their website?

If you have re-organized your website (this covers anything from changing the technical SEO to moving content around to changing the overall look of the site, and more) what lessons did you learn? Do you have any advice and stories of your experience to share?

I'm making some changes to one of my sites (menu content, the way the front page is organized, and more), so then I got curious about what prompted other people to make changes to their website. What worked for you, what didn't? Did you do something that a) luckily turned out to be a good idea or b) something that went wrong? I'm eager to learn from everyone's experiences of managing their blogs.