Sure:

• Explain your motives, something to the effect of, "I want to provide feedback so you can improve your service" or "I wanted to share my experience today so you can improve your service."
• Describe how the service could have been better, don't whine that you didn't get what you paid for.
• Compliment aspects of the flight/service that did go well.

Sometimes I will expressly say that I don't want to be compensated and they will compensate me anyway as a show of goodwill. I also prefer to share my complains over phone rather than email.