1. Use the Google Chrome Browser
2. Add the Chrome Extension - Check Them All
3. While already logged into Twitter - Goto this web Address - ManageFlitter - Click Start and authorize the app for your Twitter id
4. Click on the 'Unfollow' tab (up top) and 'Show All' (left border) - this will load the first 100 of the people you follow
5. Click the 'Check them All' button chrome extension - looks like a check mark (top right of your browser) - this will select all 100 people on the page - Uncheck those you wish to still follow
Note: the unfollow counter won't update to show you have selected all 100 users but once you click 'Unfollow' all checked users will be unfollowed. (a message will confirm this)
6. Advance to the next page and continue / repeat as necessary (daily restriction = 1,000 unfollows)
To auto follow people who follow you, you can sign up for Socialoomph and follow the config to auto-follow
Easy as 1-2-3-ish