On my site I use the plugin "Pretty Link Pro" which auto posts any new posts to Twitter with a custom shortened URL. But 3 weeks ago the plugin stopped auto posting. I have contacted the developer of the plugin who has tested all my settings which are ok but the plugin will not auto post. I presume there is a conflict with another plugin or another issue. If you can correct this issue I am willing to pay $25.
If you can't get this fixed, check out Dlvr.it. It's free, not a plugin, and does virtually the same thing.