Question: I am a product/project manager for a product we've created. We have customers using the product, which we are continually updating as it is Web based.

What are some tools and techniques for managing communications with clients regarding product updates?

Answer: 
I would suggest **to avoid** any promotional one-way communications in this particular case (and in almost every other case). Try to make the product speak for itself without a product manager behind every message. One of the methods is "[promotion through education](http://gettingreal.37signals.com/ch13_Promote_Through_Education.php)". Turn your customers into evangelists. They will promote your product.
If, as I understand from your question, you are only targeting your **existing** userbase, I would advise to use only a single way to communicate your updates. In my experience, **e-mail** with a sober and consistent format works best. Keep it brief, with only the highlights of the update, with an additional link if they wish to read more. Clearly explain the consequences (eg down-time from - until).

Three messages seems to be adequate:

* One week in advance, to inform them about the coming update (including a link with more info but also a ontact link if they still have questions)
* The day itself, as a reminder (especially if down-time or a user action will be required)
* When the system is online again (including a link for support)

In my view, attracting new users with new functionality is something completely different from informing your existing users about an update.