M60

From Manhattan, take the N train to Astoria Boulevard. Exit the tunstyles and walk down on the side of the street where the traffic is coming off of the Triborough Bridge, rather than going onto the bridge. As you come down the stairs, you'll see a small plaza with a statute; the M60 bus stop to LGA is very close to that. There are buses at 3:53, 4:23 and 4:43. The trip will be about 15 minutes.[/QUOTE]

Let me add that its perfectly safe to be there at 4am on a Saturday night. I used to lie on 43rd st and 25th Ave (about 12 blocks from the this bus stop) and spent many a drunken minutes waiting for the m60 there. You can also usually grab a cab/town car from this spot. At 4-5am expect to quite a few airport employees on the bus, as well as drunk/hungover/sleepy people.