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 like the way LiquidPlanner does it. They use the space on the right side column of the dashboard once a user logs in.

![LiquidPlanner dashboard](https://i.stack.imgur.com/ZogWc.gif)

This way whenever I log in I'll see their update and can click to find out more. If you're already using a web based system than why not take advantage of letting announcements and other news reside where users can easily find them.
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.