A WebmasterWorld thread has advice from Roger Monti, also known in the forums as martinibuster, on what he does when he is impacted by a Google algorithm change. I thought the advice was so good that I wanted to highlight it here.
Step One: Do not panic and wait for about a month to see if Google makes tweaks to the algorithm change they put into place earlier.
Step Two: When rankings do not return after a month, then look at what is ranking well in the search results and then tweak your pages to go with that strategy.
Step Three: Have an SEO friend help you by taking a fresh look at your site. Often, when you are so close to a site, you may miss big issues. So have a friend give you an opinion or two.
The WebmasterWorldthread continues on with advice from other long time SEOs on how to handle ranking changes.
Forum discussion at WebmasterWorld.