In your twitter.com account, get the embed code and paste it into a Text sidebar widget
Best and most popular (and works!)
Follow the instructions to setup OAuth with your Twitter account
Remove "New post:" in "Basic Settings > Update when a post is published"
Check "Advanced Settings > Send Twitter Updates on remote publication"
(Did not work, but looks fancy)
Adds a nice Tweet button that references your Twitter channel!
Add a Text sidebar widget with [twitter-feed] shortcode