If someone mentions you in a tweet, reply to them. If it’s positive, say thank you. If it’s negative, apologize and find out how you can make things better. If someone Retweets your post, thank them.
If your company prides itself on great customer service, make sure that flows into social media. If you don’t care about customer service, don’t join Twitter. Twitter’s a fantastic medium for generating publicity and spreading it widely, for good and bad.