I was a bit baffled to read this on the istock forums:
http://www.istockphoto.com/forum_messages.php?threadid=251222&page=1istock offered a (non-exclusive) to call and give them personal feedback, tips and tricks on how to improve their portfolio. They even send PSD's with their improvements etc! JJRD confirms in the thread its now part of the official istock program.
Thought i'd share

Im (pleasantly) surprised they're willing to put so much energy into this, and double surprised they offer this personal service to non-exclusives.