When I was taking cake decorating classes, this tip is probably one of the most valuable that I learned. I like it for several reasons. It is a handy way to store your icing and it cuts down on the mess involved in filling icing bags. You need plastic wrap.
Lay your plastic wrap down on a flat surface.
Put about a cup (or less) of icing in the middle of your plastic wrap.
Roll your icing up in the plastic wrap like a sausage and tie off the ends. This creates an air tight storage for your icing. If you didn't use milk in your buttercream, you can store this on a shelf, out of the fridge for about a week. Or... you can throw it in the fridge until you are ready to use it. This is great when you want to get your icing prepared ahead of time.
When you are ready to use the icing, simply cut one end of the "sausage" and slide it down in your bag... so easy! No mess involved. When you've used up that icing and need to refill the bag, simply pull out the empty plastic wrap and repeat the process.