Please have a look at the new best subway app: http://big-apple.tv/2013/01/21/embark-nyc-subway-new-york-city/
Whether you live in New York City or whether you are “only” a tourist, you will need a good OFFLINE map for the Subway!
Unfortunately there are so many apps in the Apple store that you will get confused which one you should download.
I’ve tried “HopStop for iPhone” which is one of the best apps for this purpose – and I’m still using it every day!
You can start at your current location and this app will show you where to go, which subway (or bus) you should use and so on.
It’s even free – so you really should try it!
If this link doesn’t work in your country – search for “HopStop” in iTunes!