If it's a HTML document, include the following meta tag in it:
<meta name="robots" content="noindex" />
This will tell Google and every other search engine that understands the tag (which should be pretty much all of them, since it's been around since 1996) not to index the page.
For non-HTML documents, you can configure your webserver to send the HTTP header
X-Robots-Tag: noindex instead. This is a somewhat newer feature, but it's understood at least by the major search engines like Google, Yahoo! and Bing.
Yet one more way to keep search engines away from a page is to disallow it in your
robots.txt file. However, there's an important caveat: Google, at least, treats
robots.txt as a prohibition against fetching the disallowed pages, but not against including their URLs in their index if they're linked from another page they've crawled. And you can't effectively combine
robots.txt with the meta tag or HTTP header based methods, since disallowing a page in
robots.txt will keep Google from ever even seeing any meta tags or HTTP headers on it.
(However, if you disallow a page in your
robots.txt and send Google a removal request for it, they'll keep it out of their index. But that's a somewhat laborious and error-prone method.)