XSLTListViewWebPart: Using CAML Query Parameters

After much searching around followed by a fair bit of trial and error, I figured out how to make use of Data View Parameters in a CAML Query in an XSLTListViewWebPart. Bear in mind I am configuring this using SharePoint designer.
So I have a list web part on a page, I open up that page using SharePoint Designer 2010 and begin editing under the hood. Once you've added a Data View Parameter you're then able to embed it as a parameter into your CAML query like so (Parameters in Bold):

<Query>
     <OrderBy> … </OrderBy>
     <Where>
     <Contains>
           <FieldRef Name="FileRef" />
           <Value Type="Text">{year}/{month}</Value>

</Contains>

</Where>
</Query>

Despite being so easy to implement, not once did I find it documented anywhere. I've labelled this post as I would have searched for it in the hope it helps out others looking for the same thing.

DataSourceID is either missing or set to an empty string

When fiddling around with the CoreResultsWebPart in SharePoint designer, or any web part for that matter, it’s easy to make mistakes in the markup.

However, more specifically, when you make a mistake in the markup of the CoreResultsWebPart it will always give the error DataSourceID is either missing or set to an empty string:

So of course, the first thing you do after seeing this is check your data source. I dragged another web part onto the page and compared the configuration with the one I’d been playing with in SPD. No difference – odd.

It was only through looking through the web part code that I noticed I had not closed a <div> tag that I’d put in for display of an image. I added the closing div tag </div> and voila! The web part no longer presented me with an error.

Having search around for instances where people were having the same or similar problem, somebody had suggested that you get this generic error no matter what’s wrong with your CoreResultsWebPart. So if you see the error and you believe the data source is intact, don’t be too alarmed – check your Web Part markup.