To make your visit to the Empire State Building as memorable as possible: 

DO:  Purchase your tickets on the official ESB website Empire State Building--Official Internet Site  before your trip.  You can print your ticket + the audio ticket on-line and these are good for one full year from the date of purchase.  On other sites, you are purchasing a voucher and you would end up having to stand in line to redeem the voucher.  Even if you go at night, the lines can still be long, but  if you have your e-tickets,  you can scoot past everyone and go straight to the elevators. 

DO:  Purchase the audio program.  It's entertaining and highly informative.  If you have kids with you (elementary school age and older), it's worth the few extra dollars and makes the visit even more memorable. 

DO:  Whether day or night, make sure you visit when visibility is good.  You want to be able to get the full impact of the City from the observation deck.