Such a broad subject, so many pitfalls to avoid.
My tip for Local growth as a business is to take part in 'local hours' Many towns and cities offer (usually run by a local business leader' twitter hours, where local companies all tweet with the same hashtag, then RT and Fav each other’s. Think of it as a digital networking hour.
If you wish for more national or global coverage then I'd suggest using the many thousands of services which will 'Tweet for you' they tweet to their audience (so choose one that has a large following) pre-defined tweets using your twitter handle. We used a service as this and gained 'real' followers, a substantial amount. However, these followers are not necessarily the demographic you require.
Ultimately, the only way you can be successful on twitter is to try and see what works for you, and your business. I would also suggest, before you go ahead and engage under your companies name, try it personally for a while, and get the feel of it. Learn about it and understand its workings. It’s surprising how hard it is (at the beginning) to tell your story in 140 characters.