The only way you'd be able to ship a backpack or bag from NY to Boston would be via UPS, FedEx or the Postal Service. Buses, trains, etc. require all luggage to be matched up with a passenger who has a ticket on the same route that your luggage is using before they will transport luggage, so that won't work. In theory, you might be able to pay an airline to treat your luggage as cargo and ship it on a flight, but that would be very expensive even if it's possible. The best thing would be to ship your luggage and have a separate backpack or something similar that just carries what you'll need for the bike portion.
I've never done it but US Route 1, which is also called the Post Road or Boston Post Road depending on where you are, should get you there. You should be able to connect up with the route in the Bronx borough of NYC (it's called Boston Road in the Bronx, but there are US 1 route signs). The URL below will give you more information on the East Coast Greenway, which is a collection of highways via which you could practically cycle up/down the entire East Coast of the U.S. It's at:
East Coast Greenway
This Bikeforums post also has lots of info:
http://www.bikeforums.net/touring/60...=#post10057843