Take the PRINCETON AIRPORTER (Van)

I got this information from P's website http://www.hep.princeton.edu/~mcdonald/travel.html

The PRINCETON AIRPORTER has an information counter at the baggage claim area in each terminal at Newark Airport. Reservations can be made at the desk when you arrive at the airport for your trip to Princeton. However, reservations must be made in advance for your return trip to the airport. The one-way cost is approximately $20 per person. The trip takes approximately 90 minutes. The shuttle service runs every hour, at fifteen minutes past the hour, from 7:15 a.m. to 10:15 p.m. Sunday - Friday.