So Posterous is shutting down. It's a shame, since there was a lot to like about Posterous, but that's how it goes with web companies. I always thought it made an impressively effective use of email to manage posting.
And that makes migration easy! At least, for some blogs, with not too many posts.
To begin, set up a new blog on Blogger (or other blog hosting service that supports posting by email, which is common). Open up your email program and find all the messages you sent to as posts to Posterous. Resend all the posts to Blogger, then go through them to clean up any problems that may have arisen with links in the posts. If you don't have too many posts to deal with, this can be reasonably quick.
This blog is the result of re-sending the small set of posts from openletters.posterous.com. The openletters blog name was taken, but that doesn't really matter—reopenletters is close enough. Since basically every post has links or videos, I had to check all of them, and actually wound up having to make some adjustments to all of them. But since there weren't too many, it only took about half an hour. Just researching to find the "right" way to do the migration would have taken longer.
The only drawback I encountered is that the dates have all changed. I could fix that, but I don't see it as worthwhile.