Long since anointed by the media (over the past year) as the NBT (next
big thing), Twitter has a bunch of local customs that people new to it
need to learn.

I've started compiling some etiquette and best practices for using Twitter,
which limits users to 140 characters. Most ideas come from other
sources, to which I will give due credit. Most of these tips are most
appropriate for people new to Twitter. (Apologies is any or most of the
tips seem familiar to people who have used Twitter for a long time
(i.e., several months).

1. Remember, Twitter is a conversation.
2. Ask questions; don't just pontificate.
   
3. What part of 140 chars do you not understand? (Thanks to jyarmis).
4. Discretion: Some things are better left unsaid. So avoid stream-of-consciousness-blogging via Twitter. Check out this article about what happened when a tweet backfired.
5. Transparency is vital -- just as with any other social media.
6. Consider quality vs. quantity. (I am going to look at what makes for a useful tweet in the next few weeks.)
7. Many users provide links to interesting articles, information, etc. Guy Kawasaki does this many times a day.
8. Use proper grammar. Check out Mignon Fogerty, the host of the Grammar Girl blog/podcast who tackles proper grammar usage on the popular micro-blogging site. Her tips are available at"Grammar Girl's Strunk & Twite: An Unofficial Twitter Style Guide," and include:
   1. "Don't
      start posts with 'I am.' You're answering the question, 'What are you
      doing?' It's OK to answer with fragments in a conversation."
   2. "Use proper capitalization. Typing in lowercase doesn't save characters; it's just lazy." I totally support this tip.
   3. "Don't use abbreviations such as 4U and L8...Shorthand symbols such as >, =, &, and @ are allowed."
      1. For an interesting look at the impact of Twitter, IM , SMS and texting may have on language, check out the Boston Globe's "Is
         language dead or evolving? Some see the use of shorthand and
         abbreviated text as the beginning of the end. But studies say students
         know the difference between formal writing and instant messaging."
   4. "Use numerals, not words, for all numbers."
   5. "If you can't say it in 140 characters, reevaluate whether you should be posting it at Twitter."
   6. "Provide links and context whenever possible. Remember that many of your followers can't see what you are responding to."
9. If people follow you, it's polite to "follow" them back. However, because
   of that, be careful about adding too many people at one time -- that's
   the Twitter version of spamming. People will think you're trying to
   sell/hype something rather than start a conversation.
10. It's really about conversation. When I tweeted about writing a blog on
    etiquette, I got a lot of response. But be careful contacting someone
    you don't know -- like in real life, you could be seen as butting into
    someone else's conversation. If they don't know you, they may not
    respond. (A better way to initiate a conversation would be to comment
    on that person's blog. That assumes, of course, that the person has a
    blog.)
    1. In fact, according to jljohansen
       in a tweet written after my first draft of this article, he said,
       "First rule of Twitter is 'Don't be creepy' after that, Right and Wrong
       are dependent on social contract of your friends/follows.
    2. To which Nedra said, "Sadly, the creepy people don't realize they're being creepy."
    3. See, how the conversation idea works?
       

Check out some other articles: Twitter + Etiquette = Twitterquette?, Twitter Fan Wiki, and "The 10 rules of Twitter (and how I break every one)" by Robert Scoble.

Of course, there may be a rule that I've just broken, according to a comment on Scoble's blog, Scobleizer: Don’t blog about how you use Twitter.