To get potential customers to sign upto your shop email what would you do?

Depends why you want the email, and what the user gets.

If it's a newsletter, then ask your customers or visitors if they want to join. If you have interesting content related to your products and services, making you known as an authority in that industry, then this can be a draw to a newsletter.

Good technique I've seen used a few times recently to get FB likes, reviews, etc. Offer a discount, or money back. Got a fiver off some tyres recently just for liking their page. Got my vote

Everyone loves promotions and offers. Use the 1+1 method. If they want to buy two products offer the cheap one for free or 50% cheaper.
If you are planning to offer something like this in the near future then you might acquire some frugal and thrifty minded clients
And if they get engaged with your business then they will spread the word. You might even appear in their blogs, comments on social shares, usually these kind of people have their own circle so if one of them finds something at a good price s/he will tell the others as well.

Thanks for the advice guys

I would recommend you to read a book such as Influence by Cialdini and I am sure you will be more aware of how to promote product/service etc.