Write the header. Include your name and, if you wish, your contact information. Also include the date on which you are writing the letter. State that this is a letter of reference for the trainer and identify him by name.
Explain briefly why you decided to work with a trainer and how you found your specific trainer. If you chose your trainer because one or more people referred you to her, mention this. It shows that the trainer has a history of satisfied clients that are willing to refer her to their friends.
State how your trainer helped you. Include physical training exercises. If your trainer offered positive support, encouragement or other intangible services, mention those as well. If you are part of a group or organization, state how many individuals the trainer trained and give examples of feedback from those individuals. Include the length of time you employed your trainer and explain whether you are still working with him.
Identify the results you saw from working with your trainer. Be specific. "Trainer Jill helped me lose 40 pounds in three months" is preferable to "Trainer Jill helped me lose weight." If the trainer worked with a team, identify any performance improvement measures like game wins or individual performance statistics.
Make a specific recommendation about the trainer's services such as "I would recommend Trainer Joe to any high school student looking to improve his batting average" or "I would recommend Trainer Joe to anyone who needs to lose weight quickly." If you no longer work with your trainer, but would do so again, say so. If you are willing to talk further with potential clients indicate the best method of reaching you.