Keep in mind that things you might be ready to discard during the demolition phase may still have value. Know this as soon as you start ripping things out and try not to obliterate all the framing material. Just like I'm not the type of contractor who's gung-ho about destroying an old house and building a completely new one in it's place, I also see no reason to throw away a stack of structurally sound 2x4's and then scoot off to the lumber yard to buy new ones. It's a waste of time, money, gas, and trees. You'll spend as much time pulling out nails as you will on the alternative; dragging it outside, driving back and forth to the store, shopping, and loading/unloading new material you'll have to haul back into the house. So save wood you can reuse. It makes sense, saves dollars, and yanking out those nails will be a good work-out for your biceps.
Copper pipes and wiring, steel, and metal will generate quick cash at your local metal salvage yards so think twice about tossing them away also. The value of these fluctuate depending on world markets and I've sometimes been surprised about what my salvaged scraps were worth. Once, I stopped by the yard with some miscellaneous metal in the back of my truck expecting to get enough money to cover my lunch tab for a few days. However, rates had spiked upward and when I checked out they gave me a payoff six times what I was expecting or enough cash to fill my truck up with gas on my next couple visits to the pump. I couldn't believe it. I was like, "What?! Really? Awesome! Thank you." All I'd done was not throw the stuff out and went a few miles out of my way on the trip back to the house.
And finally, you may not want your existing plumbing and electrical fixtures, but someone else might. (Same goes for doors, cabinets, and some trim). You can sell them yourself or at the least donate them to a charity-based building consignment house. (They're likely nearby even if you don't know about them and sometimes they'll come to you). The donation might not necessarily put cash in your pocket, but they'll surely give you a receipt you can get credit for at tax time. Plus, it's better to pass these usable things along to someone else rather than taking up space in your dumpster and then paying to have it hauled away to the landfill.
For more on salvaging and repurposing, see The Picture Window at the Bungalow and Step 4 - Pull Out the Valuables.
For more about using what you already have, see The Detached Garage at the Bungalow.