Ask any business, and chances are, they'll say that their email contact list is a very important asset.

A recent GigaOM survey of digital marketers across the country concluded that:

"Email marketing is the digital workhorse, deemed the most effective (relative to other digital tactics) for building awareness, acquisition, retention and conversion. In fact, 56 percent of respondents identified email as being the most effective at retention, several points ahead of the second-most-effective tactic."

But an opt-in list--the only kind you should have, since no reputable email service provider will let you use rented or purchased lists--is not something that materializes out of thin air. It starts small; you have to nurture it and grow it over time.

Given that I've led my email marketing company, VerticalResponse, for the last 13 years, I might know a thing or two about building an email list. Here are my 15 top faves, in no particular order:

1. For brick-and-mortar locations, the sign-up sheet at the register is an oldie but goodie. Yes, it still works, even in this digital age.

2. If your business involves physical products that are shipped, include an insert in each package with instructions on how to subscribe to your email list. Or, include a link to a sign-up form on every printed invoice.

3. Set up a fishbowl to collect business cards, letting people know that they'll get special offers or discounts if they opt-in to your email list.

4. Events are a great place to get email addresses as well, whether you're collecting business cards or having people fill out a form to join your list.If you've got an iPad, use that for people to enter their email addresses directly.

5. Feature a sign-up form on your website. State what they'll get for signing up, e.g., newsletters every two weeks, deals and steals, etc. Also, the shorter the form, the better! (Just a name and email address, for example.)

6. While website pop-up windows can be mildly annoying, if it pops up while people leave your site (this assumes they were interested enough in your company to poke around), it's actually pretty efficient. It can ask them to sign up for your offers or newsletter, which many of your readers are likely to do on their way out.

7. If you sell products or services online, ask customers at the end of their online shopping experience to sign up for your newsletter.

8. Add an opt-in/sign-up form to your Facebook page, or include a link to your hosted opt-in form and write a Facebook status to your fans every now and again, asking them to join your email list.

9. Create Facebook ads around an offer or incentive, or even your newsletter itself. Link people back to your opt-in form/sign-up landing page from that ad. Here's a step-by-step process.

10. Include a link to your opt-in page in your email signature, so people emailing with questions can learn about your list.

11. If you have a blog, include a link or embed your opt-inform at the top, side and/or bottom of each page.

12. Create a compelling image with a call to action (to sign up for your email list) and an incentive, and post the image on Pinterest and Instagram.

13. Give people a sneak peek of what they'll get by joining your email list. At VerticalResponse, we archive our weekly VR Buzz newsletter so folks can preview what they'll see before they hand over their email address.

14. If you're creating any kind of content like guides, infographics, videos, etc., you want to make sure to include a call to action to subscribe to your email list at the conclusion of the content or somewhere in the body.

15. Incentivize your employees. They're some of your biggest assets, so why not give them a little something extra for gathering opt-in email addresses?

Got any tips or tricks that worked for your company? Share your secrets with me in the comments below.