We store ours in a plastic shopping bag (once it's cooled) so it's pretty airtight. And I make either a small loaf, usually 750g which gets us a sandwich each for one day, plus toast for the kids the next morning. I find it's so quick and easy to bake that I don't mind baking a loaf every day or so.

So that's my advice. A plastic shopping bag (twisted closed), and only make as much as you need for a day or so.