The best way that I've found (I've got a brand new replacement MacBook for my old IBook) is to write to them directly at their offices. For me in Europe that address is:
Holyfield Industrial Park
Cork, Republic of Ireland
If you live in the United States, try this address:
Apple Computer, 1 Infinite Loop, MS60-DR, Cupertino, California, USA, 95014
Don't try email, you may as well toss your complaint in the ocean. I emailed three separate times to a specific Apple employee, I didn't get ONE response by email.
One of the best ways to complain is to say that you have not been subject to Apple's well known and much lauded high customer satisfaction and service. Who would want to disappoint someone who is complaining and complimenting you at the same time?