Twitiquette

Do's

Do add some content as well as a link, if you're posting a link: There is nothing worse that seeing a tweet that contains nothing but a link or worse a tinyURL. TinyURL is great but it gives no clue to what is on the other side of the link. Will it be a rick role, goat.se or a link to free beer. Try to give a clue.
Do keep it short: Use contractions, symbols and numericals wherever possible. if you can't say it in 140 characters, reevaluate whether you should be posting it at Twitter.
Do feel free to have more than one account: If you have a tool that auto posts content, or updates from your blog or you're going to a specific event and you'll be posting huge amounts. You can always start a new account and allow people to follow that one specifically. Not doing this and you run the risk that people stop following you at all, due to the volume of content that they don't need to see.
Keep it Current: If you are going to reply directly to a person, try to do it in a timely fashion. There is nothing worse than someone who twitters every other day, responding to someone who twitters 50 times a day about something they said a week last thursday.

Don't Try to Follow thousands of people: It immediately looks like spam and people who you try to follow will probably think you are a bot or spammer, try to focus.
Don't Overuse @: You don't always have to be talking to someone, in fact constantly addressing other people can become tiresome. Try keep @ for a real person to person interaction, or in response to a direct interaction.
Don't be redundant: Starting every tweet with "I am" or similar is very redundant. We know it's you, you're posting it has your user name next to it.
Reciprocation of following isn't essential: Just because someone follows you doesn't mean you have to follow them, not following someone should not be seen as a bad thing or a personal attack.
Don't assume people read twitter: "Dude where were you? I've been trying to get hold of you"
"did you try my cell phone"
"No, but I've been posting on Twitter all afternoon"
Not everyone is checking Twitter every 30 seconds, some people might only check in once a week, if you really need to talk to them, why not just give them a call or get up from your desk and walk over to them.