Hello
Build relationship
Interact with people who follow you or whom you follow. Talk to them, answer their questions, listen to what they have to say, if you want to ask something - ask don't shy, tell them about yourself, if you have read some book and liked it. Recommend that book to others. This way you will build relationship and building relationship plays an important role in business promotion.
Don't Spam
Don't keep on sending same message again and again. One of the common mistake people do is keep sending same message again and again, thinking it will make them popular and get sales. But just opposite happens, people get irritated and stop following them.
Keep your status updated
Its important to keep your status updated with latest happenings in your company. Your customers would like to be updated about the company. But it doesn't mean if new product is coming in market, you keep on twitting about it after every 10 minutes.
Don't talk only about your product
Don't keep on talking only about your product. Talk about other things also, no one will be interested in listening only about your product. People use twitter to talk and interact with others. Don't start telling about your product as soon as someone comes and say hi to you. Be polite, say hi, learn about them, build relationship, tell them about yourself and then you can talk about your product.
Be interesting
Keep posting interesting stuff. Post things which will be of interest to other users whether its your company's news, some general topic talk or advice on something.
Reply back
If someone has asked you something, retweet i.e. reply back. Answer their question and respect the feedback you get from others.
Find prospects
You can find potential customers online by using Twitter Search. Do keyword search to find potential clients, customers and people from your industry and follow them.