Offer an incentive such as a coupon or discount for them to add themselves to your email list. Once you have your list, you can offer specials mailing list that only your list will receive. They are your "preferred buyers list". Treat them well, and they will come back time and again to do business with you and give you their money. Let your mailing list know exactly what services you offer.
You would be surprised at how many customers will come back to you and say, "I didn't know you did that" or "I didn't know you sold that". Keeping your existing customers informed about what you have to offer is so easy, so profitable and yet mailing list so few businesses actually make the effort. Once you start mining your list for repeat sales, you can easily expect to increase revenue every time you send an email out to your customers.
This is also a good way for you to bring some traffic to your business website. Processing orders online is more cost effective for most businesses than making an on-site sale from your place of business. If your customers know they can mailing list shop with you 24/7 you can be processing orders when your business is closed, while you are sleeping and even on Bank Holidays.