Right now I have a very small XSL file that operates on a fairly big XML doc. Using:
<xsl:template match="//page[@id=2010]">

As the root of my template results in the XSL giving me just what I want. The problem is I want 2010 in the example above to be a parameter I pass in, so more like

<xsl:template match="//page[@id=$pageId]">

Which I can't do. What's a good way to do this in XSL?

Assuming your code is doing nothing with the other elements in the document, move the logic to the apply-templates level

<xsl:template match="/">
  <xsl:apply-templates select="//page[@id=$param]"/>
</xsl:template>

If you are processing different elements in different ways, then you will need a template rule with match="page" containing an xsl:choose, xsl:when test="@id=$param".

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference