This is what Wikipedia has to say about “Negotiation”. And I totally agree to it.
Negotiation is a dialogue intended to resolve disputes, to produce an agreement upon courses of action, to bargain for individual or collective advantage, or to craft outcomes to satisfy various interests. It is the primary method of alternative dispute resolution.

Negotiation occurs in business, non-profit organizations, government branches, legal proceedings, among nations and in personal situations such as marriage, divorce, parenting, and everyday life. The study of the subject is called negotiation theory. Professional negotiators are often specialized, such as union negotiators, leverage buyout negotiators, peace negotiators, hostage negotiators, or may work under other titles, such as diplomats, legislators or brokers.

Thirty-Three Rules to Remember:

Thirty-Three Rules to Remember which I read from a book called- Start With NO / by Jim Camp

1 Every negotiation is an agreement between two or more parties with all parties having the right to veto - the right to say “no”.
2 Your job is not to be liked. It is to be respected and effective.
3 Results are NOT valid goals.
4 Money has nothing to do with a valid mission and purpose.
5 Never, ever, spill your beans in the lobby-or anywhere else.
6 Never enter a negotiation without a valid agenda
7 The only valid goals are those that you can control: behavior and activity.
8 Mission and Purpose must be set in the adversary’s world; our world must be secondary.
9 Spend maximum time on payside activity and minimum time on nonpayside activity.10. You do NOT need it. You only want it.
10 You do NOT need it. You only want it.
11 No saving. You cannot save the adversary.
12 Only one person in the negotiation can feel okay. That person is the adversary.
13 All actions and all decisions begin with vision. Without vision, there is no action.
14 Always show respect to the blocker
15 All agreements must be clarified point by point and sealed the times (3+).
16 The clearer the picture of pain, the easier the decision-making process.
17 The value of negotiation increases by multiples as time, energy, money, and emotion are spent.
18 No talking.
19 Let the adversary save face at all times.
20 The greatest presentation you will ever give is one your adversary will never see.
21 A negotiation is only over when you want it to be over.
22 No is good, “yes” is bad, “maybe” is worse.
23 Absolutely no closing.
24 Dance with the tiger.
25 Our greatest strength is our greatest weakness (Emerson).
26 Paint the Pain.
27 Mission and Purpose drive everything.
28 Decisions are 100% emotional.
29 Interrogative-led questions drive vision.
30 Nurture.
31 No assumptions. No expectations. Only blank slate.
32 Who are the decision makers? Do you know ALL of them?
33 Pay forward.