When you first started twitter, you may have felt overwhelmed fitting everything in to one little 140-character tweet. Abbreviating where you can, leaving off some of the details and trying your best to just cram it all in and still have room for a link should be considered a skill. Well I hate to burst your bubble but now you have to do it in less characters.
One of your twitter goals should be to get retweets from your followers. In order to make this easy (see How to retweet properly), you need to leave enough characters so retweeters don’t have to edit your content. Specifically, subtract the amount of characters in your twitter name + 5 to save space for “RT @[yourname]” at the least. This doesn’t allow room for the retweeter to add a comment so leaving off a few more characters is even better.
Don’t like doing the math? Just make it a general rule to use a maximum of 120 characters per tweet.