I usually wait just by the crossing to the yellow cabs and ask people if they're going to (for me) Lower Manhattan and fancy sharing a cab. I usually get a taker within 5 minutes if it's at a time when the terminal is busy.

Generally there are only going to be a couple of points where the cab is going to cross the river, so sharing can be quite an easy option for most people.

If I'm in Manhattan and have a friend arriving at the airport, I jump in a Zipcar and go and get them. I think the cheapest cars are $8 an hour all inclusive (including fuel!) + they have an EZpass built in so you don't have to stop at toll booths (if you go through any).

So if your girlfriend fancies that, zipcar is a great value option. There are parking bays close to the airport where people can wait for a call from their incoming traveler for free.