Reputable local content. There is no restriction on this, but make sure it has been cleared with the appropriate authority (PAO or whoever has this responsibility).
Follow those you wish to have follow you. This will tell them you exist! For example, if you want to build a media following, follow the major press outlets. If they follow you, Direct Message them informing them about upcoming trip-related Tweets. Or any Tweets that are key messages.
Search the "following" lists of others to get ideas for new groups or organizations.
Take advantage of the DoD Leaders twitter list, the Pentagon Press Corps twitter list, or the DoD Twittererlist, which include a variety of principles, senior officers, and other service members who tweet.
Going to be using a unique hash tag for a special event? Tweet it! And ask others to Retweet it.
Don't bombard your followers with tweets. Unless live-tweeting a major event, 10 tweets max per day, and spread them out.
Adopt a voice. Don't sound too formal, but stay professional. Let your followers know there's a real person behind that embassy name!
Have an interesting image on your profile, not just your service seal.
You don’t have to tweet from Twitter.com. You can use a program like HootSuite.com or TweetDeck.com so you can track your stats.
Remember: Twitter is a conversation. Retweet, ask questions, respond to tweets, answer questions, and have fun!