One of my favorite things to read is Craig Ball’s writings at Ball in Your Court. I have hit Craig up in the past as he has a unique perspective as an expert in computer forensics, e-Discovery and is a lawyer. He has been super helpful, and I can’t recommend him enough.
He wrote a paper called, The Perfect Preservation Letter, and I have given it out a ton.  He originally wrote it back in 2009, but has since updated it. Here it is!