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: 
If the updates to the product affects the customer's ability to use the web application, I will encourage you to use your time of less traffic and to give ample notice. 

If you are only thinking of the actual strategy of communicating the updates, I will suggest the following:

1. Create a YouTube campaign with videos explaining the benefit of each update. This will help educate your users as you tell them the good news.
2. Create a webminar type seminar were you will show users that subscribe the new updates. This might seem like an overkill, but you will get questions from the users that you did not think.
3. Send twitter messages that encourage people to take action, update your profile before the update, etc. you get the idea. This will let you know how effective is that medium.

You need to send similar messages from different sources.
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.