Here's a quick, easy, inexpensive way to seal foods airtight in a plastic bag. All you need is a zipper-lock bag, and a tub or pot of water.
As far as plastic bag storage goes, air is the enemy of food. It causes oxidation in the refrigerator, it can exacerbate freezer burn in the freezer, it can promote more rapid spoilage of raw ingredients, and it can slow down sous-vide cooking. Removing the air from a plastic bag is also a great way to distribute marinades around food. It's simple to do with a vacuum sealer, but what if you don't own one or don't want to use the expensive bags for a relatively simple storage or cooking task?