Store-bought ice cream stays softer when it's frozen because of all the chemicals they add to it. I think Xanthan gum is the thing they add to keep it soft. Here's an excerpt from WikiPedia:
Xanthan gum is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris, used as a food additive and rheology modifier,[2] commonly used as a food thickening agent (in salad dressings, for example) and a stabilizer (in cosmetic products, for example, to prevent ingredients from separating). It is produced by the fermentation of glucose, sucrose, or lactose by the Xanthomonas campestris bacterium. After a fermentation period, the polysaccharide is precipitated from a growth medium with isopropyl alcohol, dried, and ground into a fine powder. Later, it is added to a liquid medium to form the gum.
Now that makes me want a big old bowl of that stuff. I agree with Carol Dee above and eat your homemade ice cream all in one setting. Call me and I'll come over and help...