It is a common view that the optimum length for tweets should be the maximum length of 140 characters (including spaces and punctuation).  Wrong.

A key element of the success of twitter is the extent to which information can be passed on, or the use of the retweet as it is commonly known.  Despite the recent introduction of a native twitter retweet function, the retweet or RT was created by users and the original method ‘RT @username’  is still the preferred means of retweeting.   If other users are to retweet your microposts without editing them, there needs to be space left for them to do this within the 140 character limit.   Therefore the optimum length of a twitter message depends on the length of your username and the simple equations is as follows:

x = 140 – (y+5)

In this formula x is the optimum length of the message in characters and y is the number of characters in your username.  The five is the number of characters required for using RT @, including the space – plus an extra space before the original tweet (thanks to The Crocodile for pointing out the need for that final space).  Geektatsic.