Getting the field names right when creating a Word Mail Merge template to use with Salesforce Extended Mail Merge can be frustrating and time consuming.
However, if you navigate to a record in Salesforce, e.g. a Lead, the url will be something like
https://cs99.salesforce.com/00Q99000002Kwwp
Now, if you paste 'servlet/servlet.SForceMailMerge?id=' after the 'com/' so that it looks like
https://cs99.salesforce.com/servlet/servlet.SForceMailMerge?id=00Q99000002Kwwp
You get a (not very pretty but very useful) list of the Mail Merge fields that you can copy and paste into your Word doc.
Thanks dacology.