Twitter can be a major time waster. Don't get me wrong...I love the application.  However, if you don't know how to use it correctly it can be a serious distraction.  TwiTip covers the topic in a post here and they make some excellent points (except for batch Tweeting....I can't stand getting 20 tweets from someone within a span of an hour).  In addition to some of the recommendations they mention, here are ways that I keep Twitter from ruling (ruining?) my day:

• Turn off notifications in Tweetdeck and other apps that notify you every time a tweet is received.  Do you really need to waste your day monitoring what all 342 people you are following are chatting about?
• Engage in stuff worth engaging in and don't waste time with mindless chatter.  Tweeting like a high school kid with ADD drinking Red Bull will probably end up getting you followed by the same crowd.
• Don't feel like you have to respond to all DMs and @ replies.  Let 'em go, man. Not every response requires you to have the last word. 
• Don't follow people you don't really care about in hopes of getting a return follow (AKA "ego-follow").  It only clutters your feed with - you guessed it - crap you don't really care about.

I know everyone uses the tool differently - some for business, some for personal, and some for a bit of both.  But no matter how you use Twitter, setting some rules for yourself can keep you from letting a useful tool turn into a time drain.