First and foremost, its use. I have chosen, and would advise others, to use Twitter as a business tool. To be your business-self while on it. To think of it that way - as a business mini-blog. This means, since I assume you are a therapist like me and like some boundaries and privacy, that you don't twitter your personal matters on it (that great date you had, the annoying client you had, etc. etc.).
I would contrast this to Facebook. I made a decision at one point to make Facebook a business-free zone. Since I work with a lot of therapists, I actually had to de-friend some people (there was some sadness on both sides about this, but it felt like a good move). Facebook is built to allow privacy, and really is not a great business resource (for therapists, at this point), and I think we therapists need a place where we can be silly and stupid and just people and not have to watch what we say/think/post.
OK, back off my soapbox. Since Twitter is going to be business tool, I suggest you pick a username that is either 1) your name (Peter Hannah), or 2) your service + your city (SeattleTherapist). I established my username before I had figured out how I was going to use Twitter. Don't make my mistake. But do realize that Google is now adding Twitter into it's search engine results. For instance, go search Google for gratefulguy. Guess what's the first entry? Moi.
Oh yeah, and to show the power of this thing - guess where I first found out about this new (it's quite new) Google/Twitter thing? From http://twitter.com/jeanlucr @jeanlucr on Twitter. Here's the actual news item - http://news.cnet.com/8301-17939_109-10203095-2.html