I'll trim just a small corner off the bag. Hold it by the top and corner straight across from the trimmed corner. Begin shaking the bag to level the contents before it gets to the pour out position. This leveling prevents the cloud of DME dust that happens when the bag is just up ended.
Slowly squeeze some of the air out a partial bag for storage. Fold the cut corner over and secure with a binder clip. Small bags I will put in a gallon zip lock bag for added security.