So, you may have noticed that some bar soaps are individually wrapped when you buy them. This wrap keeps the bar moist and it causes the soap to get used up faster. At least that's the theory, I haven't actually timed it, but it does seem true.
So, to keep your soap lasting longer, simply unwrap it before you store it. This will help it to harden and last longer when finally used.
I store my soap in my linen closet, so the smell of the unwrapped soap helps it to smell nice, too.