I was advising a student today who had received a job offer, and thought the pay offered seemed kind of low. I suggested to her that she do some research on salaries, and then ask for more.
Many people don't ask for more in fear that they might jeopardize the offer. I can tell you that I have never heard of a company revoking an offer because someone asked for more money. Further, I've never heard of a company or manager who thought poorly of a new hire that had tried to negotiate for more money. In fact, most companies expect you to ask for more. They aren't always going to give it, but they expect you to ask.
Before you do, make sure you do your homework. I think http://www.salary.com/ provides some good information. I also suggest looking for local salary surveys, or salary surveys for your industry or profession for more information.
Another mistake I see in the salary negotiation game is with people negotiating a reduced hour work schedule. Some are so grateful to get the part-time schedule, that they take what we call a "part-time penalty" in their pay. Your pay reduction should be in line with your work reduction. If you reduce your hours 20%, your pay should be reduced 20%, and not any more. In fact, in some cases you can even negotiate for a higher hourly rate. If your reduced schedule takes you to a part-time status no longer eligible to receive benefits, I suggest asking for an hourly pay increase to make-up for any lost benefits.
The best way to be prepared to negotiate salary is to educate yourself. Here are some links to some good articles on the salary negotiation process: