8 characters to 6:
tweet…
Replace words with symbols that mean the same thing
Replace words with pictures
Use ligature characters where available
Shrink smiley faces
Replace (3), (b), etc.
Replace 13. etc.
Replace numbers as words with digits
Remove final full stop.
Attempt to eliminate redundancy
Apply common contractions
Replace simple fractions
Use common shorter synonyms (use Thsrs for more)
Attempt to shrink punctuation
Desperate measures

Other tips: Use Thsrs to shrink long words. The shortest URLs are generated by tinyarro.ws. If you absolutely must, use TweetShrink to shave off the last few letters while sacrificing readibility. Please be careful, because at present this tool will mangle @usernames, #hashtags, and URLs.

About: I made this because I got bored of copy-pasting the ellipsis character every time. Yes, I'm lazy in very specific ways. Created by @Andrew_Taylor. Any problems or suggestions, get in touch.