There are many paid and free  twitter apps allows to automate DMs and follow people on twitterYesFollow  allows to automatically send DMs to new followers and auto-follow people those follows you,  you can also schedule tweets using this service also.

YesFollow access to your twitter account through Oauth, so you never have to fear about giving your password. For every hour, this service checks for new followers for your twitter account and automates following those people follows you.

You can also send auto DMs to new followers, you can schedule and publish tweets even without  your need in front of Computer. This service uses URL shortening service and allows to track number of clicks for shortened links.

YesFollow is free twitter application.