I think the best thing you can do is ask to speak to the supervisor/purser and tell them you'd like them to send a compliment to the crew's manager. I've sent compliments about my colleagues and luckily I've received compliments from my own manager who happened to be on my flight. That could help them with a prospective upgrade.
But the little compliments on disembarking also made my day (in the past, because I'm now resigning). Comments such as "Thank you for the lovely flight", or "Thank you for the amazing service" along with a genuine smile etc. They're always welcome.
Another thing really touched me was when pax showed genuine concerns towards my wellbeing, such as asking after the service if I had eaten already. Or if I had had my rest. Little things, great impact.
