<xsl:for-each select="//client[not($clientid) or @ID=$clientid]">
<xsl:for-each select="descendant-or-self::client">
<xsl:for-each select="usage[@TOTALSENT > 0]">
..... do something here....
<xsl:for-each>
<xsl:for-each>
<xsl:for-each>

It would be really great if I could just do.....
<xsl:for-each select="### super select that does all that! ###">
..... do something here....
</xsl:for-each>

<client ID=1>
  <client ID=2>
    <client ID=3>
      <usage @TOTALSENT=5/>
      <client ID=5>
        <usage @TOTALSENT=10/>
      </client>
    </client>
  <client>
  <usage @TOTALSENT=9/>
  <client ID=4>
  </client>
</client>