How do you increase the contacts names on your email list for marketing of your products ?

You can create a facebook page and add a registration form there. Each contact registering should get a gift voucher. 

This way, you will not only add contacts to your database, but you will also add interested customers in your business. -Laith - Client Support Manager at Emarsys Email Marketing

You can buy or rent email lists.

Adjust your sales procedure so that the customer's email is collected every time you make a sale.

Adjust other procedures so that every time anybody comes in direct contact with the business their email is collected as a routine part of the contact.

Social media can be used, although it's often better to continue the relationship on social media rather than moving it to email.

For a long time great success was had by combining pay per click advertising with a "squeeze page" which obtained the prospect's email.    Google has made this much harder now, but the principle is still good.  It is common to give some value in exchange for the email, the standard example being a free report.

Your main website should also collect emails: it may in fact be the primary purpose of your website.