Bring as little as possible. The marathon officially begins at 9 a.m., but chances are, unless you are an Olympian and/or experienced marathon runner, you will have a much later start time (along with 20,000 other folks. And don't feel bad, all the fun of the NYC marathon happens in the later waves). This means that you will likely get to Staten Island a few hours before you line up for your start. November weather can be tricky, and even if the day turns into a beaut, as it did last year, it will still be cold in the morning and on the ferry, so bundle up.
The marathon has introduced new baggage options this year that will basically allow you to (theoretically, anyway) skip the bottleneck at the finish line if you chose not to check a bag. DO NOT CHECK A BAG. Whatever you wear to the start area, make sure it's something that you are willing to ditch along the way. Your best bet is to go to the Salvation Army and pick up some cheap clothes that you can then dump into the recycling bins the marathon folks are setting up at the start gates. Believe me, the very, very, very last thing you want to be doing after finishing running 26 miles is to be standing in line in Central Park waiting to pick up the bag you checked six hours earlier because it has your favorite Nike running top in it.