Bookmarklet: Abbr. This will open Abbreviate and import any text from Twitter's tweet box, saving you a copy-paste. From any other webpage it will open Abbreviate with a blank tweet box.
Other tips: Use Thsrs to shrink long words. The shortest URLs are generated by tinyarro.ws. is.gd do the shortest ones with only common characters in. If you absolutely must, use TweetShrink to shave off the last few letters while sacrificing readability. This tool will never alter @usernames, #hashtags, or URLs.