I did this by getting the value of the querystring:
Code:
<xsl:param name="Selection">
<xsl:choose>
<xsl:when test="/root/QueryString/color"><xsl:value-of select="/root/QueryString/color"/></xsl:when>
<xsl:otherwise>0</xsl:otherwise>
</xsl:choose>
</xsl:param>
Then within the drop-down, you will want to do an if to mark the selection:
Code:
<option value="{VariantID}">
<xsl:if test="Color= $Selection"><xsl:attribute name="selected">selected</xsl:attribute></xsl:if>
<xsl:value-of select="Color" />
</option>
This is just an example using color as the option. I actually used the variantID to do the selection. Its up to you though.