1.) Interesting people - Many people will use apps to remove people who do not follow back. I don't agree with this, not everyone has the ability to keep up with thousands of people but are more than willing to engage if you message them directly. If they are interesting, follow them, interesting people provoke interesting tweets from you. Also, you loose important amplification potential if you remove those who don't necessarily follow you back.
2.) Engagers, Reply or Re-tweets - Has the person engaged with you before? If they have they probably will again, every time you engage, you increase your reach and amplification. If someone re-tweets me, I always check out their stream and see if they are posting value, if they are, I will follow them. This is a great way to increase your clout in the areas in which you are confident and you'll probably make some friends on the way. ALWAYS good.
3.)Friends of Friends - The thing about twitter is, if the first character in a tweet is the @ sign, you will only see the tweet if you also are following that person. If that tweet includes multiple people at the front of the tweet, then you wont see that message unless you follow all of them. By following the followers and friends of those you enjoy chatting with you ensure that you won't miss out on moments when you can contribute relevantly to the conversation. Imagine that twitter is a Venn Diagram. You want to be in the middle so you don't miss out.
4.) Businesses - People tend to underestimate twitter accounts representing a business. A lot of the people behind these accounts are personable and energetic. Some of my first exposure on twitter came when Fred Meyer (A grocery chain) re-tweeted something I said about a sandwich I got a local Fred Meyer deli.
Really, its about following people that you find worthy of following, if you don't find them worthy of your valuable follow, don't follow them. Sound easy enough?