For a good while I was frustrated with the subject line of the workflow e-mails within SharePoint Designer.  Where as in the body you can combine look-up fields that pull in data from the list item along with free form text, in the subject line you can do only one or the other.  If you put a look up field in the subject line, it greys out the field so that you cannot enter any more data.

 

 

The easiest way to get over this hurdle would be to us the action 'Build Dynamic String'.  This allows you to store a combination of text and look-up fields, just like you can within the e-mail body, and then store it as a variable. 

 

 

You can then set this variable as the subject line within the e-mail by clicking on the function button to the right, and then selecting Workflow Data as the source.  The field will then let you select from all of the variables you have set.