What's the best way to center your HTML email content in the browser window (or email client preview pane)?

Align the table to center.

<table width="100%" border="0" cellspacing="0" cellpadding="0">
    <tr>
        <td align="center">
            Your Content
        </td>
    </tr>
</table>

Where you have "your content" if it is a table, set it to the desired width and you will have centred content.

For googlers and completeness sake:

Here's a reference I always use when I need to go through the pain of implementing html email-templates or signatures: http://www.campaignmonitor.com/css/

I'ts a list of CSS support for most, if not all, CSS options, nicely compared between some of the most used email clients.

For centering, feel free to just use CSS (as the align attribute is deprecated in HTML 4.01).

<table width="100%" border="0" cellspacing="0" cellpadding="0">
    <tr>
        <td style="text-align: center;">
            Your Content
        </td>
    </tr>
</table>

table align="center" ... this aligns the table center of page.

Using td align="center" centers the content inside that td, useful for centered aligned text but you will have issues with some email clients centering content in sub level tables so using using td align as a top level method of centering your "container" table on the page is not the way to do it. Use table align instead.

Still use your 100% wrapper table too, purely as a wrapper for the body, as some email clients don't display body background colors but it will show it with the 100% table, so add your body color to both body and the 100% table.

I could go on and on for ages about all the quirks of html email dev. All I can say is test test and test again. Litmus.com is a great tool for testing emails.

The more you do the more you will learn about what works in what email clients.

Hope this helps.