From musicradar.com, “27 engaging Twitter tips for musicians” that are great advice for non-musicians as well — anyone hoping to use Twitter for professional or business purposes would benefit:
1. Have an objective
What do you want to achieve? If you want to widen your fanbase by building a Twitter community around yourself, talk about making music and eventually try to take over the world, you’ve come to the right place. Knowing what you want is half the battle, stick to the following and you’ve a great chance of achieving it!
2. Be descriptive
Choose an appropriate username, don’t call yourself @TheDarkKnight if your band is called Loose Change. Provide as much information in the bio section as you can, believe it or not, people will read it, as you should read theirs!
3. Get to know your audience
People fill their bios in for a reason, read them, see what they’re into and talk about it. That way you will!
4. Engage with your followers
The golden rule. Don’t just talk about yourself, take an interest in everyone else. Reply to messages and be as personal as possible. If it’s good enough for Trent Reznor!
Read the full post at musicradar.com.
(Thanks to Venangago-gofor highlighting the post.)

Great blog. Do you know of any relevant marketing forums or discussion groups?
Good question, John. Most of the discussion I’m aware of regarding how best to use Twitter happens on Twitter itself. Or rather, the discussion hops around on Twitter and then people contribute longer ideas and theories on their own blogs, and link to them on Twitter.
For people just starting out in the social media world, I recommend reading Chris Brogan’s blog. He frequently recommends other super resources, so anyone who starts there and follows links and threads is going to come up to speed quickly.
Can you recommend any forums and discussion groups?
There is obviously a lot to know about this. I think you made some good points in Features also.