At our house, the heels of the bread loaf are not super popular. So, when we get to the end of a loaf, I'll add the heels and any bread that may borderline stale to the plastic gallon baggie I keep in the freezer for this purpose. Once I've got enough heels collected (and my crouton supply needs to be replenished) I pull them out to thaw. I preheat the oven to 350 degrees, cut the bread into cubes, and toss into a big bowl. Melt some butter or margarine (about 1 Tbs. per cup of bread cubes) and pour it over the bread. Stir. The cubes should look damp but not drenched.
Spread the bread cubes out on a baking sheet and sprinkle generously with garlic salt. When it comes to garlic salt, "more is more," as they say, so don't scrimp. You could always use different seasonings, but I like garlic salt because it's super-flavorful and super-cheap. Bake for 20-25 minutes, stirring periodically. They should all be crisp and browned slightly.
Allow to cool, then store your croutons in your repurposed jars. We eat a lot of salad, so I've never actually had the opportunity to test this out, but I think they ought to stay good for at least 2 weeks.