You can automatically tweet your new blogposts using Google Feedburner, but tweeting all your old posts that has not been tweeted yet is a time consuming process. Specially, if you have lots of them and doing it manually.
If you use self-hosted wordpress, you are in luck. There is a plugin called Tweet Old Post. It is a plugin developed to tweet your older posts automatically by a cron job.
Tweet Old Posts randomly picks your older post based on the interval specified by you. The primary function of this plugin is to promote older blog posts by tweeting about them and getting more traffic.
Once you install and activate the plugin, you will see a new settings group named “Tweet Old Posts” at the left sidebar of your wordpress admin dashboard. You then need to setup the plugin to start working.
To get started, you first need to authorize your twitter account by clicking on “Add a Twitter Account”. This plugin uses oAuth to authorize and login your twitter account. Once it gets authorized, you will see your profile image at the plugin setup page.
There are several other settings like Tweet Prefix, Add Post data to Tweet, URL Shortener Choice, Hashtags etc. But the main settings are setting the interval. A Cron job actually takes a lot of server resources, so you need to enter intervals (in hours) wisely. Entering 2 hours as the Minimum Interval and 1 hour as the Random Interval is okay with almost all the servers. However, you can choose to use the default interval, which is 4 hours.
You can also choose to exclude certain categories and posts as well. Excluding a category will prevent the plugin to tweed all the old posts from that category.