Here are some ideas for using that wonderful invention, the zip-locking bag. These bags can be used for freezing leftovers, including various stocks and gravies, fresh fruits, and vegetables. Even if the identifying label falls off in the freezer, you can see what's inside. Not so with opaque plastic containers.
Another use is for marinades. Simply put in a piece of fish or meat along with your favorite marinade and squeeze out all the air as you seal the bag.
Correctly done, the marinade should surround the food evenly, and it will not be necessary to turn meat every few hours as usually suggested.