I'm no expert on revisions, so I turn to respected authors/bloggers for guidance. In particular, Janice Hardy's blog posts on revision and her weekly Real Life Diagnostics series have been extremely educational. Here are some helpful links:
Janice Hardy's posts on Revisions
How to Revise a Novel
Before I send out anything to my CPs, I do at least one revision on my own. Then, after getting critique, I'll do another one or two before sending anything out to Beta readers. Generally, after I receive feedback from Betas, I make smaller changes which I wouldn't really consider revisions (more like tweaks).
I haven't gotten to the next phase of revisions yet - I dream of receiving a revise and resubmit request from an agent and can't even begin to imagine the amazing feeling of receiving an editorial letter or copy edits - but there is so much information available on author blogs (like Rachelle Gardner's breakdown of the Editorial Letter) that I'm sure I will have a pretty good idea of how to handle things should I ever reach that point.
What about you? Do any of you have experience with agent-requested revisions or editorial letters? Do you have any recommendations for great resources for writers diving into revisions?