Great piece by Mark Suster:
The problem with negotiating piecemeal as Stuart taught me is that you trade on every item. You don’t prioritize the issues which you really care about. If you don’t want to give a millimeter on one item you have a hard time doing that point-by-point. Done as a “package deal” you can say, “I gave in on these 5 issues that you asked for. On this issue I can’t give.” That’s much harder to pull off piecemeal.
Piecemeal you might be reasonable in your negotiation on each of those first 5 issues as each came up. You found middle ground on each of them. When the 6th point comes us – the one you really care about – you’ve lost your leverage. You might have been better off not finding middle ground on the first 5 points but rather having given completely on all of them in exchange for not budging an inch on point 6.
Chalk it up as one of these "obvious once you've been told" ideas that can save you a lot of money and worries.