Tipping in advance is an uncommon approach with frequent dividends. I picked it up years ago from a book. It has rarely disappointed, especially in Asia. if you really like the service and it won't break you, no harm in another, smaller tip at the end. It's the easiest way I know to add a star or two of service at your hotel.
That often can be said of mamasans if you expect to be a regular. I apologize for a stereotype, but there seem to be a number of ill tempered katoeys running the works at establishments. They perk up considerably with this system, save for those who sadly never will.