Here are some of my favorite Twitter tips from Missing Manual.
140 Unicode characters
The length of a tweet is limited to 140 characters, and unicode characters count as one character. For most this would be assumed, but if you know how Unicode characters are stored, they are usually two or more bytes, so you may assume that they would take up more characters in a Tweet. This is not true.
Twitter direct messaging
Prefixing any tweet with a d, m, or dm, will cause Twitter to treat the tweet as a direct message. This means that the next word is treated as a username, and then the rest of the tweet would be a direct message. This can come in handy when you want to send a direct message quickly, you can just start a tweet with the aforementioned words and then the username of the person along with the text of the message. If you use Twitter over SMS text messaging, this comes in handy when sending direct messages. But also, be warned, if you reply to a direct message over SMS and do not include d, m, or dm and a username, the tweet will be posted to your timeline.
When replying to a tweet, try to use the reply button in your Twitter client. This way Twitter will know which tweets belong together, and can format a tweet and its replies correctly. The person to which you are replying can also see the thread of the conversation, helping the person know what you are talking about. If you start a new tweet with @USERNAME, the person will get a notification, but Twitter will not be able to know if the tweet is connected to other tweets and be unable to group them.
The username to whom you are replay has to be mentioned in the reply, but it does not have to be at the beginning.
A tweet that begins with an @USERNAME will only show for your followers if they also follow @USERNAME. That is why you will see some Twitter users put a period before the @, such as .@USERNAME. This way it will show for all of their followers. On replies, the mention of the user has to be in the tweet, but not at the beginning, so you can move the @USERNAME to the end of the message and then all of your followers will then receive your reply regardless of whether they follow @USERNAME.