I've written content for a variety of different types of publications and media in the past and also have led design efforts across multiple organizations. Both of these efforts have benefited from some form of guidelines, best practices, patterns, and conventions to help contributors create the highest possible quality. I believe the Twitterverse could similarly benefit. I should point out, though, that some people may feel that the informal nature of Twitter makes this type of material unnecessary and possibly even antithetical. However, I've received enough suggestions, both solicited and unsolicited. about Twitter's unwritten rules that reinforces the need to write these things down.

Twitter is growing at an incredible rate and some best practices and conventions appear to be emerging in certain areas. Yet other areas could benefit from some common approaches. The number of company Twitter accounts is increasing and there is little guidance for the people who are tweeting on behalf of their company. Consistent with Twitter's focus on brevity and informality, I'd like to keep these guidelines brief and informal too.

Let start by settling on a few terms: a "tweet" refers to the 140 character or fewer piece of text that is sent on Twitter; a "retweet" is a forwarding of a tweet to others; a "reply" is a response to a tweet that is viewable publically, and a "direct message" or DM is a response or communication that is private.

Content. You can write anything you want but think about what might be most interesting to those following you. Think about what you enjoy most from other people's tweets. If you're tweeting on a company account, make sure to cover a variety of topics in your discipline area and avoid exclusively discussing your own company.
Format. Write as if you're having a conversation using the first person with the people who follow you unless you are tweeting on a company account on which it is better to use the third person but still use a conversational manner. You can respond to someone publically using the "@" sign or privately using a "d" in front of their account name. Most of the Twitter tools have options for replying or direct messaging (DM) which automatically create the appropirate @ or d format.
Length. Everyone is aware that the length of any tweet cannot be longer than 140 characters. However, if others want to retweet what you've written, it is a good practice and a nice courtesy to them to use far fewer characters in your tweet so that the retweet will fit within 140 characters. A good rule of thumb is to leave 20 to 25 characters of retweeting room.
Attribution. A "retweet" is the technical equivalent of the word-of-mouth sharing or passing on of information. Communicating attribution or whom you got the information from is critically important in the Twitterverse as it is in areas like academic publishing. If you want to pass on something someone else said, make absolutely sure to include their Twitter account name in the tweet you send using the reference format of RT @ followed by the person's account name at the beginning of the tweet. Another popular format involves using "via @" followed by the account name at the end of the tweet. There is no set convention for adding a comment to a RT but I would recommend putting your comment at the beginning of the tweet and separating your words from the retweet by "->".
Indexing. Hashtags which are created using a "#" key followed by a short word or acronym are used on Twitter to provide a tag for the topic or topics you're tweeting about so that others can go directly to a tag like #design and be able to read all of the tweets from a variety of people on the topic of design. Hashtags still aren't pervasively used likely due to the fact that they take up precious characters in the 140 character input field so it is a good practice to simply search on keywords too in order to see what people are writing on that topic.
Evolution. Twitter is still new and is evolving at a rapid rate so be attentive to changes in how others are tweeting, be responsive to comments made by people following you about your tweeting habits, and most of all, have fun!

This is my brief summary of what I think may be of value to twitterers. Please use the commenting capability of this blog to provide any feedback or other suggestions of your own or send me a tweet using @karelvredenburg or the company account I tweet on @ibmdesign.