Find street parking. It may sound crazy, but we were able to do it in July 2008 when we were in NYC (or I should say, DH was able to). We had a rental minivan, and the quote for parking was something like $60/nite, so DH circled around for about 20 min and got a spot 3 blocks from the hotel. We had to be very careful that followed instructions, etc., for street parking limits, but I think the total cost of feeding the meter was around $5 for the 2 days.
(DH is from Chicago, and is very good with street parking in the city, which helps).