In this question, Duopixel gave me a lot of very useful feedback, much of which I used to improve my answer.
I have been flagged as helpful a couple of times by other users, but I'm not sure how to flag people myself; so far as I can tell, I can only flag their comments as unconstructive or worse. I'm obviously missing something here; have I misunderstood what "helpful" flags are for, do I need more reputation, or is it something else entirely?
(If no other avenues exist for me to reward him, I might just check out some of his other answers and upvote my favourite ones.)
Update: Thanks all, it seems that I misunderstood the flags system. I was under the impression that I had been flagged as being helpful, but it was actually my own flags (i.e. for moderator attention) were deemed helpful. You use a flag to indicate that there is a problem, and that flag is helpful if you make a good call.