Get one of poll boxes with a slit in the top and put it on the counter. Beside it put a tray that holds little paper forms where folks at the store can put their name and email address and drop them into the box. Simple and quick.
Hang a few banners around the store that list the benefits of joining your email club. These should be really convincing like "Special prices for Club members, not available to the general public."
At the counter... encourage people to signup.
This doesn't mean you shouldn't retail on the Internet too. But first... assuming your Dad has a proven customer base... try to get them into the store a little more often.
Best Regards,
Steve MacLellan
http://carsite-marketing.com