Use template rules:
<xsl:template match="/">
<html>
<body>
<h2>Lesson Plan</h2>
<xsl:apply-templates/>
</body>
</html>
</xsl:template>
<xsl:template match="concept">
<p><xsl:apply-templates/></p>
</xsl:template>
<xsl:template match="math">
<i><xsl:apply-templates/></i>
</xsl:template>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference