I did this recently....
...In Atlanta. I applied 3 months before my EAD was due to expire and it was approved and issued less than 1 month later!
Just ask the INS for a new form by mail(this is better than downloading it from the net because there is a fingerprint form as well that they will send that is not downloaded). Fill out the form, specifying that you are renewing, get your photos taken, fill the FP form and send the whole lot off with a check and then wait for your appointment.
By the way, I applied for my extension at the local Atlanta INS office because that is where my Lawyers filed for my 485.
Good Luck.