I try to keep my twitter feed relatively clean - I don't follow too many 10k+ tweeters, and I'm pretty selective anyway. My signal/noise ratio tolerance is pretty low. Tweet about your cat too often, of how you've just had a shower and are considering what to have for breakfast and I will unfollow without much guilt or hesitation.
One new trick I've learnt is the selective unfollow. Clearly, in a small world like librarianship, especially in England, a lot of people know each other personally. Therefore, if you follow groups who know each other, you will see all of their @replies to each other. This is fine if it's either occasional, funny or professionally relevant. However a lot if it is at best noise in my stream and at worse tweeters ostentatiously showing everyone how social they are.
To combat this, identify either the weak link in the chain (the least interesting tweeter) or the worst abuser (most frivolous @replies) and unfollow. You then get to keep most of the relevant tweets from the group and hugely cut down the amount of chatter in your stream.
I know this sounds a bit antisocial, and I'm all for using twitter to communicate with my friends, but when it becomes too obnoxious or ostentatious I'm gong to start looking for reasons to unfollow pretty quickly. Also, if someone is interesting enough, their best tweets will be retweeted by someone else in my timeline and I'll see the best bits anyway.