To Follow via SMS:

For instance, if you'd like to follow @Oprah and get her Tweets delivered to your phone, send a message to your Twitter code with the command 'FOLLOW username', like so: FOLLOW Oprah

If you do not follow @Oprah already, this command adds her to the list of people you follow and sends all of her Tweets to your mobile phone. If you already follow her, the command will simply make sure you get her Tweets on your mobile.

To Unfollow via SMS:

When you want to stop seeing Oprah's Tweets, simply turn updates off for her by sending the commands "LEAVE username" or "OFF username" to your Twitter code, like so: LEAVE Oprah

You will no longer get her updates on her phone. However, you will still be following her on your web profile. This means that if she sends you a direct message, you will receive it. You'll need to log on to your web profile on www.twitter.com to fully remove her from your following list.