In the past when I have asked a lot of the hotel concierge pre-arrival, I will usually bring a gift/souvenir from home along with a hand written card for them upon arrival.
The last 2 trips it has always been a bottle of Canadian ice wine, and the concierge has always been super appreciative, and usually gives us extra attention during our stay to make sure everything is going smooth with our stay.
For my trip coming up, I'm thinking of doing the same for Amanoi and Aman Tokyo where I've been in contact with just one individual, but for the upper house where the entire concierge team has been helpful, I will likely bring a box of Chocolates from Canada along with a hand written card for the team.
I find this to be a much more sincere way to show my appreciation than just slipping them a $20 or $50.