I have tried most wordpress twitter plugins. They seem to work great for awhile and then all of a sudden just stop. Very frustrating.
But if you use Feedburner for your RSS feed there is now a super easy and reliable way to have your new blog posts automatically sent to twitter.
One you have logged into your Feedburner account, go to the Publicise tab, and then select the Socialise option from the side menu.
Then just fill in your details for which twitter account you want the posts published to. You will be redirected to twitter to authorise feedburner to publish to your account. Click activate at the bottom and you are done. Any new posts will go to twitter automatically, its usually quite quick within about 5mins of being published.
If you have some old services tweeting your posts and are unsure how to stop them. You can block their access to your account by going to the connections tab in your twitter settings. In here you can revoking access to the services which you do not wish to access your account.