If you get great service from an FA, I'd say write a letter or an e-mail the airline's customer service and let them know what specifically you appreciated about their work and in what way said excellence would motivate you to keep flying with them.

If you get attitude, laziness, etc. than I would similarly write or e-mail the airline, conveying your understanding of the challenges FA's face, AND objective, succinct, non-emotional descriptions of what was improper. A note of what the "proper" way to have done things could have been, or the proffered manner of handling things. (posit a solution, not just the problem). I don't think the threats of "Ill take my flying elsewhere" really get far, unless you're the head of a corporate travel office with substantial airline revenue generation under your thumb.

In wording these, I'd say the shorter the better, stick to the point, eliminate emotional & ranting lingo.