My husband and I have made four joint trips to New York City, and for the first three trips never set foot on a subway. This caused problems like excessive amounts of walking, crazy taxi budgets, and once getting stuck in Lower Manhattan during a cabbie shift change and trying to walk 70 blocks north in time for a Broadway show. After that experience, I swore I would not return to New York until I had mastered the subway.
Luckily for me, HopStop mastered the subway for me. HopStop is a free (with ads, paid for: ad-free) mobile app available for many flavors of phone and as a mobile website as well, that gives you directions to get around over 100 cities via transit, taxi, car rental, and hoofing it.
For NYC, and I assume other cities, you enter where you are (it will find your current location if you allow it), where you want to go, and HopStop tells you how to get there. My favorite part was that it gave walking directions to the relevant subway station, several options for trains to take, instructions on where to get off, and then walking directions to your destination. It was awesome. I used it constantly for four days and it was entirely accurate every time, even when it came to changes and closures caused by Hurricane Sandy. If you are planning a trip to an unfamiliar city, or even a familiar city you haven't visited in a while, I highly recommend this app.
Image credit - hopstop.com