How to improve my Analytical Writing skills for the GRE?
I don't have any specific sites or resources but I have some advice,

Write, write, write, and write some more

Get a notebook and set aside 40 minutes every day to write a sample essay. A list of topics can be found on the ETS.org website (so I guess I have one resource :) ).

Then analyze and grade your own essay. Compare to the template, to any available sample essays you find, etc.

The best way to improve writing is to write.