7) Design smart and plan ahead.
Before you start picking out materials, create a design and plan a budget. Consider limiting the following: changes to load bearing walls, moving sinks and stoves, and changes to electrical and hvac components. If you are thinking of changing the layout or adding windows, consider the sunlight throughout the day and any views you may or may not want to see. If you measure twice and plan ahead, you will reduce mistakes and waste.
6) Donate used items to salvage shops and go shopping while you're there.
There's no reason to throw away old hardware, light fixtures, or cabinets. Call a salvage store to schedule a pick up or sell items on craigslist. You will be surprised how fast old hardware sells at a garage sale! Some salvage stores like Habitat for Humanity's Re-Store will give you a tax write-off. If you're at the salvage shops, look around for materials and hardware you need. Some stores even have brand new tools and supplies.
1) Reuse what you already have!
Just because you're removing walls doesn't mean you can't patch existing floors. Just because you're installing recessed lights doesn't mean you have to take down the whole ceiling. Many contractors want to demo as much as possible so they can start over with a smooth and level surface, but at what cost? Old houses are imperfect and that old plaster ceiling is more soundproof and insulated than new drywall will be. Often this step is more expensive than buying new products and throwing out the old ones, but reusing existing items gives your home a sense of history and is the best sustainable decision you can make.