What I did not know until recently is how to set a variable to an element. So what I did before was to hack in the source a piece of code like:
<ns0:countryCode> <xsl:value-of select="$landCodeNL" /> </ns0:countryCode>
It turns out that you can do that by "drag-and-drop" as well. In the component-palette you need to select the "Advanced" functions:
When you edit it you can just type in your expression, for instance just a variable. When you start with a dollar sign, it even gives you a drop-down-list with available variables. Just pick the right one and our done.
I admit, no high-standard tip, but convenient enough though, for me at least.