When you are ready, there are some things to consider when choosing a replacement. New startups are still finding their legs and could be bought or go under, but if you don't mind the uncertainty, they often have some of the most exciting new designs. Look for sites that offer ways to import your old data, and if you crave stability, for companies that are profitable and still growing.
There are many good alternatives to Posterous vying for the attention of the newly homeless bloggers. WordPress is established and dependable, Tumblr is easy and attractive, and newer companies like Medium are exciting, but their futures are uncertain.
If you can wait a bit, two Posterous co-founders are working on a new project called Posthaven. They pledge never to sell the company and to keep all URLs online no matter what. "This one is made to last forever," reads the site's tagline.
Forever is a pretty bold promise online. Make a backup, just in case.