Last year I was getting requests for surveys after all of my flights, so if I was getting exceptional service I would always ask for the FA's name and I would put it in my survey. Depending on the situation if you get their employee number you can always use that as well.

There is also a place on their website where you can do that, it's somewhere in "contact us" or something like that. I used it last year on one of my flights that was really really good, and it was staffed by a friend of a friend. I got their employee number through my friend, and I even got a response back from United saying that they were going to recognize them - never followed up to see if anything came of it though.