The PANYNJ will sell you an off-base parking permit for LGA. The employee lot is 10E which is adjacent to the marine air terminal (sometimes called "the MAT"). The permit is about $185 annually. Just go to the main terminal, 3rd floor, room 3575 with your company ID and your checkbook. Tell them you're an off-base pilot and need a parking pass for lot 10E.
JFK is another story. There are no off-base passes available. You're better off taking the LIRR to the Airtrain if possible. You can also attempt to park on the street near the Howard Beach Airtrain station (station A) or on Lefferts Blvd across the Belt Parkway near station B.
LGA to BOS is a piece of cake. You've got the US Airways shuttle and the Delta Shuttle. JetBlue does JFK-BOS.
For LGA-DCA you have the shuttle. There are a few flights from JFK to DCA too, but LGA has much greater frequency.
USAirway's Express does LGA-PHL several times a day.
DFW is going to be to be on American from LGA several times a day with several connecting options. A few from JFK-DFW on AA too.
DTW will be NWA from LGA. You also have some connecting options. There's also a couple of NWA Airlink flights from JFK-DTW.
The following are great sites to check flight schedules:
www.panynj.gov/aviation/oagframe.HTM (for NYC flights)
www.passrider.com (for all flights)
Hope this helps.
Tipsy