I only took the AP portion a couple of years ago, and here was what I did...
Questions:
Lefkowitch's Review of Anatomic Path
Sternberg's Review Book (a question book, which the difficulty and format most matches the boards)
Robbin's question book (didn't get through all of it)
Images:
Robbin's CD (comes with the textbook)
Sternberg's CD (comes with the textbook)
That's all I did, got about 60% right on Lefkowitch, which is the most thorough review, and I studied the answer explanations to all of them (ie. KNOW at least Lefkowitch cover to cover) Didn't crack open another book or look at other slides. Didn't do Osler either. Wouldn't look at study sets..unless you don't know what's on the image CD's...if so, then look on pathologyoutlines.com, or google images, or the Hopkins site.
I also went to almost all of our daily slide unknown conferences and took advantage of our awesome AP education (ie, pay attention during your years as a resident!). Felt that I comfortably passed during the exam (didn't know everything, but knew enough).