Unfortunately, you can only extend your warranty with Applecare once and you must do it within the first year of owning the machine. Once you're outside of Apple's complimentary year, you're sort of out of luck.
Now, if you NEED some sort of protection on your machine you could check out
http://www.safeware.com/ which sells independent laptop insurance. Rates are pretty reasonable and they even cover stolen/damaged machines.
Hope that helps!
Kyle