The arts blog Lies Like Truth references the "pitfalls of institutional blogging" (here) which I think is relevant to the conversation of blogging for promotional purposes.
The point they are trying to make and that I agree with is that it is important to be positive, educational and productive when posting. It's the best and really the only way. Otherwise, you are wasting your own time and the time of your client.
Thou Shall Blog takes that a step further (here) by stating that you only have about ten seconds to interest your reader. So make it succinct and worth reading.