If you have a true job offer in hand, the first thing you need to do is decide whether the offer is acceptable to you in its present form. In other words, if the offer as given is the very best you can negotiate, will you still accept the job? If not, you will need to take a different approach.
In either case, it is always important to know who at the company controls the offer. Not just making the hiring decision, but also making the decisions about the offer components. This is usually the Hiring Manager, but not always. Hiring authorization may actually come from a level above the Hiring Manager. There may also be input from a Salary Administrator or Compensation/Rewards Consultant in Human Resources, although they are usually there for input, not for absolutes. The key is to know who makes the final hiring and offer package decisions. If you don't know, ask. Ask the Hiring Manager, the person to whom you will be reporting. Remember, it is always in their best interest to make this happen. Now that they have made you a job offer, you have one foot inside their company. You have access to information you did not have prior to the offer.
There is an easy way to determine whether an offer is acceptable. The method was originated by one of the wisest men in the history of the US: Benjamin Franklin. The name of the technique? The Balance Sheet Decision Technique. He probably never called it that, but good old Ben used it just the same. In weighing a difficult decision, he would take a sheet of paper and draw a line down the middle. Then he would put the heading "Reasons For" on the left side and "Reasons Against" on the right side. He would proceed to list the positive aspects of the decision on the left and the negative aspects of the decision on the right. Once everything was down on paper, the answer usually became obvious to him. Use the Balance Sheet Decision Technique in evaluating the initial offer. Important: do not evaluate the position against what you want. Instead, evaluate it against your next-best alternative. For some, that may be your present job. For others, it may be another job offer. And for others, it may be unemployment and continuing your job search.
Whatever you do, don't get caught in the "hope trap"—comparing your offer against what you hope another company will offer. Until you have it in writing, you are comparing against vapor. Make sure all of your negotiations are on firm, solid foundations. Otherwise, you may find it slipping out from under you. Objective number one is to get the job offer. Then, and only then, can you begin to negotiate that job offer.
If the job offer is still truly "unacceptable" per the Balance Sheet Decision Technique, continue with the Unacceptable Offer Negotiation Technique which follows. Otherwise, what you really have is an acceptable job offer that you were hoping would be better. Read the Acceptable Offer Negotiation Technique for advice on how to potentially improve this type of offer.