1: Interact with Followers
Don't be that company who sends out a bunch of tweets, but ignores everybody who tweets you. Not only does it make the follower feel worthless and unwelcome, but you could miss out on important things they have to say. Feedback is a vital part of the communication process and an invaluable tool to help learn about what you are doing right and wrong. Granted, replying to everybody might be out of the question, but interact with as many as you can. If people see you don't talk to anybody, you look lame.* Don't be lame.
2: Tweet Relevant Things Relating to Your Business
This might sound obvious, but don't tweet about random stuff that has no relation to what your business is about. Just because you sell sheet metal does not mean it's ok to tweet about death metal. People who follow your company care about your product, not your scary taste in music. While we're on the subject of relevancy- whoever does the tweeting for your company should be somebody who understands your products and knows how to communicate well (or can be taught both). For example: you don't want your college intern to flirt with the owner of a business you sell to via your company's Twitter account. That could be bad- very bad.
3: Ever Heard of the Social Media Business Equation?
It's important as heck to ask for business via Twitter, but don't act like a hobo. Asking every moving organism for money will give you a bad reputation and customers will shy away. Eve Mayer Orsburn, CEO here at Social Media Delivered, came up with a pretty nifty equation that maximizes the marketing potential of social media. Equations can be scary I know, but I failed math my first year at university and even I can handle this. 20% of your tweets should inform your followers about your business, products, etc; another 20% should be entertaining, engage your followers or else they get bored and leave; and still another 20% should convert to business because you obviously need to sell your stuff. The largest chunk is 40% interacting with your followers, because see * in Section 1 above?. Put all the parts of the equation together and BOOM- dolla dolla bills, y'all.