Five things you should not say while negotiating:

1. “Between”, don’t use a range in your offering
2. “I think we are close”, it’s a sign of frustration and willingness to compromise
3. “Why don’t you throw out a number?”, be the first bidder
4. “I’m the final decision maker”, always leave some room for advice from your dog!
5. “Fuck you”, I say use it if needed

Source [Inc.com]