We spent the entire fall in the throes of a major house reno. It was tons of fun, everything worked out well, and I learned a great deal. The most important lesson was the golden rule of construction: measure twice and cut once.
This is smart advice to take literally but it's also a wise principle to apply more generally. Spend a lot of time planning. Review your work. There will be fewer mistakes and when surprises do crop up, you'll be better prepared to deal with them.