There are many suggestions on who to follow on Twitter, enabled also by tools like Twubble and Twitdir.But as far as I know there are much fewer posts about un-following. Here is my take.
Recently I started "cleaning up" my Twitter account. I remember my initial enthusiasm which ended in following more than 1,500 people with no criteria. The only criterion I remember was: "the more, the better." After a while I asked myself: what am I using Twitter for? To stay in touch with people. About what? About my interests.
What are my interests? Adobe Flex/Air programming, Apple, iphone, music, geekness. In the past, when I added people I checked whether her tweets were about one of these topics. Recently I started to watch with more attention the tweets appearing in my home. Posty helped me in this process. Here is a list of questions which helped me filtering Twitterers by looking at their tweets.
What language is this?
When I stumbled upon a tweet with "strange" characters or words I opened that profile and then asked: does she regularly post in this language? If yes then unfollow. Moral: how can I be interested in what is expressed in a language that I don't understand?
Is she active?
I usually remember pictures, even if they are small like stamps, like those of Twitter. I started opening less "popular" avatars appearing in my home to check whether a Twitter was active or not. To check "activness" I asked: how frequently does she post? I ended up with unfollow unfrequent Twitteres, those who did not update the profile in one week, those who posted once in the last month, etc. Moral: follow those who are present and active.
Does she reply?
You might be in front of a very frequent Twitterer but ... does she post, post, post without replying? Try to figure out some replies/post ratio. If it tends to zero you are in front of somebody who just wants to "tell", "be there on the internet", "promote her service", etc.
Moral: try to follow thos who are "interactive". Isn't Twitter a social service? Then it is a matter of relationship, sometimes you say, sometimes you listen and reply. If you always say, sorry but you are not for me.
Is her content interesting?
If the posts of a Twitterer are mainly about knitting and you hate knitting why are you following that Twitterer? (no offense to all the lovers of knitting :). In this case the profile of the Twitterer can help. If she is a html/css programmer it is likely she will post something about that sometimes. To find it out, again, check the last tweet.
Are the graphics the default ones?
If a user does not personalize at least a bit of the layout this might be a symptom of the behavior: "I have Twitter though I do not use it".
Following / Followers ratio
This can be a delicate topic. Without getting into math formulas, it is intuitive that if a user has 23,000 followers, she is famous. If, at the same time, she follows 10 people, this might indicate that she does not care much of following back her audience, or have a look at a fresh home each time she checks in Twitter. It is also likely that she just publishes tweets and replies very rarely.
A more balanced ratio signals a more equilibrated participation to the "life in Twitter".
If, on the contrary, you follow more than you are followed (e.g. follow = 10 and followers = 1), this can be the result of the attitude: "If I follow I get followed". To some extent this is true: there are people who tend to follow back by default. But if you start an account from scratch and follow 2,000 people I bet you get followed back by 200 at most.
How many updates? How frequent?
There are companies out there with a really inactive Twitter account. I think it was created to follow the boom of Twitter, to say "I am there", but with idea in mind probably. If you see that the last update is from six months ago you probably do not want to follow this user unless you know him personally and you are aware she is in a long busy period.
These guidelines have been elaborated with the goal of raising up the Twitter grade of my Twitter accounts. The tools I have found useful are friendorfollow and Posty.
Posty 1.7 was released today, and you can find it here: http://spreadingfunkyness.com/posty/