This is a great tip i got at a workshop. The lady used it with royal icing but it can be used with buttercream also. She grabbed a big freezer bag, and pulled one of the bottom corners through the coupler in a piping bag. Then she cut the tip, and folded it over the coupler so that the freezer bag would be tightened when the tip and the screw was put in place. She then put the icing in the freezer bag, and tied it up, and then pulled the piping bag over it. No icing escaped, and when it was done, the whole icing bag was still clean, the freezer bag can be thrown out, and you can move on to another colour without having to wash your bag.
I hope it makes sense, kinda hard to explain without seeing it ; )
Nati