Hooray! You received an offer! (or offers!) Now, it's time to consider a few key factors:
Entry-level salaries are often not negotiable in larger organizations since new hires may be part of a group of entering employees. In smaller organizations, there may be more room to negotiate salary and other potential benefits. Your decision to negotiate should be based on data such as market pay for the positions (as discovered through the websites listed above) and a reasonable budget based on the actual cost of living you will experience. See the PayScale Salary Negotiation Guide for more information.
It is acceptable to ask for more time to consider, but you need to be specific about your reasons for seeking an extended deadline for your decision.
Swarthmore Career Services has developed a great guide on managing offers and salary negotiation, including sample scripts.