Posterous has introduced ReTweet buttons that would make sharing posts on Twitter super-easy. To disable or enable the ReTweet button on your Posterous blog go over to your Manage page by clicking here. Then click on the Settings tab, where under the Look and Feel section click on More Theme Options. There simply select Enable Retweet Button.
You can also add the retweet button and customize its look on your custom theme. For that, please visit the retweet documentation for the Posterous theme engine here.
So enjoy the new ReTweet button on your Posterous blog and make your posts famous!