I asked this exact question a few weeks ago, and the good Flyertalkers of the NYC forum helped me out.
http://www.flyertalk.com/forum/new-y...-purchase.html
When you get to Jamaica, as you get to the ticket machines, turn around and look behind you to your right. Those two ticket machines sell combo Airtrain Metrocard + LIRR tickets with no Metrocard fee, just $5 more than the cost of the LIRR ticket alone.