To automatically follow new followers on Twitter, you can simply use the service SocialOomph.com:
Once you've registered (it's free) and added your Twitter account, go on Social Accounts > Edit Automation
Then tick Automatically follow people (new followers) who follow me from this point forward.
Et Voilà, now all your new followers will automatically be followed :)
For more info on Twitter, see the summary Twitter: how does it work?