On the original question--the best way is indeed to use the "Comments" link on the Delta Website. List flight number, date, and employee name (or position if you don't know their name--it's enough, they will figure out who it is). Also include your FF number and your status (it makes a difference--apparently higher status members are harder to please?).

I've found there are a handful of key words or phrases that get the attention of their supervisors. I've found noting the employee went above and beyond normal actions to help me, and described the specifics. I also explain why this type of behavior reflects very positively on the airline. If appropriate, explain how this incident will influence your coming back to Delta.

With this type of wording, the FA will certainly get an 'Attaboy" from their supervisor and potentially some other rewards.

By the way, the same advice still holds if your comments are negative--if that type of situation ever comes up.