I've been flying AA out of LGA for years and always seem to end up in Concourse C where there is no AA lounge. Its usually not worth it to go through security at D to get to the lounge and then have to go through security again at C to board the flight
A few weeks ago, we found ourselves with two hours to go before our flight so we headed on over to D and got into the AA lounge with my platinum card (works for another few weeks). The agent noted my gate and informed me there is a service started a few months ago that will shuttle us directly to the gate without having to re-do security. 25 minutes before the flight a LGA employee met us in the lounge, directed us through the bowels of LGA out on to the tarmac where a shuttle bus drove around to C and let us into the secure area through another back-door. I thought that was pretty cool.
Make sure the front-desk agent is aware so she/he will tell you when to meet at the front desk.
