Your plan, upon receiving a job offer, is to thank the employer for the offer and ask for time to review all elements. You should never either accept or reject an offer immediately. If not given the offer in writing, this is also a good time to ask for all the details of the offer in writing.
Once you have the written offer and you decide it's not exactly what you want, choose just a few items to attempt to negotiate. Review one technique for negotiating here: Job Offer Too Low? Use These Key Salary Negotiation Techniques to Write a Counter Proposal Letter.
Note, while some employers will state upfront that they will not negotiate salary, they have often very open in negotiating other aspects of the job offer, such as job title, start date, early salary review, bonuses, life insurance, vacation days, relocation allowances, profit-sharing, and the like.
You have only one shot at negotiating a better job offer. It's not like buying a car where you can go several rounds with the salesperson until you get the best deal. In job-hunting, the employer makes an offer, you accept or reject or counter with a modification of the employer's original offer, and the employer then accepts (completely or partially) your counter or rejects it. There is no other round of negotiations.
Again, the lesson here is to take time to review the original job offer and then decide what, if any, aspects you will attempt to negotiate.
(Whatever you do, do NOT have unrealistic expectations about your worth in the marketplace -- or you will never be satisfied with any job offer you receive. Use these salary tools to assist you.)
In fact, some employers with great compensation packages sometimes offer lower salaries... so even if the salary is a bit lower than you expected, examine the value of the total package before dismissing the offer. For example, review our Comparing Two or More Job Offers.
Finally, remember that at any point in the negotiation, the employer may either withdraw the offer completely or attempt to force you to stick to the original offer. If you decide to negotiate, you must do so with the mindset that you can either walk away from the offer and/or risk losing it entirely.