I'm fortunate to fly BA long haul twice a year escaping the English winter. If I consider that I have been given particularly good service then I always make a note of the crew member(s)' name & after I arrive home fill in the online thank you form. I'm pleased to report that I've filed such reports on 95% of flights covering 12 years.
I'm even happier to report that I've never had to file a complaint.