Sounds like you have your answer already, but I just wanted to mention that I just did the reverse of the above subway/bus combo last week, worked great. And I have tried almost every possible means of transportation (except for the Delta ferry and the helicopter). It took about an hour and you can't beat the cost.
http://www.panynj.gov/aviation/lgrtfram.htm

Also believe it or not, if you ever want info about other airports, you can actually Ask Jeeves this question: "how do I get from Manhattan to laguardia airport" which is how I learned about this.