Controlling whitespace in the output is often a problem. A solution: remove all whitespace and then add whitespace back in as required

<xsl:variable name="newline"><xsl:text>
</xsl:text></xsl:variable>
<xsl:variable name="space"><xsl:text> </xsl:text></xsl:variable>
<xsl:variable name="tab"><xsl:text>&#9;</xsl:text></xsl:variable>

<xsl:value-of select="$newline" />
<xsl:value-of select="$space" />
<xsl:value-of select="$tab" />

Calculating values for reuse several times. You need the absolute value of an XML fragment:

<num>-620</num>

<xsl:variable name="absnum">
 <xsl:choose>
  <xsl:when test="0 > num"><xsl:value-of select="0 - num"/></xsl:when>
  <xsl:otherwise><xsl:value-of select="num"/></xsl:otherwise>
 </xsl:choose>
</xsl:variable>