I've learned a few things about taking rooftop pictures in New York. The easiest way to get up is to have a friend in the building (in the absence of a friend, simply walking in usually works-- it pays to be friendly, reasonably well-dressed, and armed with a cover story.) Once in, take the elevator to the top floor, and find the nearest stairwell. Most staircases in New York terminate at the roof, and the fire department requires that they be kept unlocked. Often they'll be one of those panic bar assemblies-- ignore them. 99% of the time they are turned off. Don't touch the panic bar-- just turn the doorknob and walk out on to the roof. I recommend timing your visit early or late in the day to get the best light.