If you are in the service business (and who isn't?) you should take a minute and read: 12 Things You Should Never Say To A Client.
Yeah, I've said most of these things before (d'oh!).
The only one on the list I'm not sure I agree with is #8, "Of course, you can call me any time." I'd rather be connected and talking to a client then making them feel like I'm not available. For me, the benefits of good communication have always outweighed any potential inconvenience in terms of timing. And when you're talking frequently, clients tend to be understanding if you're not around at a particular time.
Still, it's an excellent list and one worth taking the time to consider how you can fold it into your business.