- What the person did for you;
- In concrete ways (time/money saved; income/opportunities increased; psychic benefits accrued); and
- Why you would work with them again. (Because they are timely, accurate, effective, easy to work with, adaptable to situations, your client/customer was thrilled with the result.)
Then, put it in writing in no more than a few sentences. Send it in an email, letter or card to them. Give it in person to everyone you can. Post it on review sites, Facebook, LinkedIn, etc. They will be glad you did.