How do I sign a letter of recommendation, if I've left academia?
In your case, write on personal letterhead.  As Alexander Woo has said, the first paragraph should be something like this: &quot;I'm writing in support of the application of Bob Brown to your doctor of divinity program.  Bob was my student when I was a postdoctoral fellow at the Close Cover Before Striking University.&quot;  Then you go on to explain briefly your qualifications to make the recommendation and in more detail why Bob would make an excellent and successful DD student.