This is basically a no-brainer. If you are not some kind of dominatrix be thankful for your viewers and tippers –and tell them! If they tip you well and you thank them for it, they will more likely tip you again – either in the same session or the next one. This plays well along with the point that you should smile and be friendly to your users. No matter if you are one, but at least play the nice girl. It will come to your benefit.