Retweet, retweet, retweet
Retweeting does exactly what it says on the tin - the Twitter equivalent of clicking 'forward' on that email round-robin that's going around the office. By encouraging your followers to retweet your updates, and by judiciously retweeting others', you can push your @username out further into the wild - netting you all kinds of new followers, not to mention the goodwill of those you retweet.