In my mind nothing beats a letter on paper sent to WHQ, next best is an email to UA, and third best is a GTEM certificate. I know of people who split the difference and send an email, then print a copy of it and mail it to WHQ.

When I put something in the mail, I send it to:

Ms. Sara A. Fields
SVP People
United Airlines
1200 E. Algonquin Rd.
Elk Grove Township, IL 60007

I never hesitate to ask for an FA's or GA's employee's name and ID number to reward good service. Usually they're happy to disclose it over if they understand why you're asking. I usually say, "I'm very impressed by your service ethic and service spirit, and I'd like to send a short note to United to let them know my thoughts if that's OK with you. Would you write your name and employee number on a napkin or a ticket jacket?" I have done this a number of times and have never been turned down.

When I write to UA, I usually explain what I've observed (assuming, as WayMaker said, it's not something that's likely to get the employee in trouble), give the employee's name and ID, mention the origin and destination stations and flight number if applicable or mention the station if it's an airport employee, talk about how the employee's actions are unusual in my observation as a very frequent flyer, and encourage UA to recognize the employee for making a positive difference in my travels.

I did this recently for a UX FA who had a tremendously uplifting personality and an obvious interest in serving passengers, and she was overjoyed when I spoke to her (in fact I initially was worried that she was going to start crying or give me a hug right in the aisle). It was a great feeling to know I'd made her day.