Open with a short, but enthusiastic, bit of praise. Let the company know right off the bat that you believe in this person. You don’t have to be over the top or insincere, but a positive note to start will make a big difference. It makes me extremely happy to recommend Michael for the position of Director of Development at XYX Corporation. Any company should count themselves lucky to have an employee as bright, friendly, and dedicated as Gina. No matter what she does, Helena Bonham does it well.
You put your own reputation on the line when you write a letter of recommendation. If you feel you do not know the person well enough, or they are somebody you cannot in good conscience recommend, decline their request.