Email campaigns and social ads can help you move the needle and attract new customers online. But for many small companies, digital advertising will never fully replace the value of word-of-mouth marketing -- or good old-fashioned face time.

In fact, a simple thank-you card or a friendly offline introduction can lead to new business opportunities you didn't know were on the table. The best part? If a current customer is the one recommending your services, there's an element of trust already in place.

The most successful founders always look for new ways to impress current customers and attract new ones for less money.

Below, six entrepreneurs share their most cost-effective offline marketing strategies.

1. Always be closing.

Whether you are in the airport, at a family vacation or your local coffee shop, always be talking about your business.

You never know who you might meet or come across when actively talking about your business in everyday life.

2. Be part of your community.

Your local community can sometimes be the best thing for your business. You can't beat a word-of-mouth recommendation.

Be neighborly, frequent local community events, participate in neighborhood functions, go to your kids' soccer games, host a potluck, organize a block party, etc. Just get involved, and socialize! Simply being out there can actually result in some of the best networking opportunities.

3. Send a simple thank you card.

After a meeting, a cup of coffee, a phone call or a proposal, send a simple thank-you card in the mail.

More often than not, I get emails thanking me for the card I've sent, and we have been told that they have made the difference in the proposal process.

4. Make introductions for clients.

Making introductions for clients is a powerful offline marketing strategy. This could mean making a personal introduction to people they want to collaborate with, new resources or tools that can simplify their business, articles they'll find useful or more.

It's a personal touch that tells your clients that you're thinking of their business and their success every day.

5. Apply to speak at every event you can.

Since my company often doesn't have the budget to pay for expensive conferences, I come up with creative speaking proposals and try to use that as a way to get into the conference and get in front of an influential audience.

6. Volunteer.

Volunteering your time and expertise to a community organization is a powerful way to make new connections and showcase your skills.

Local nonprofits are always looking for knowledgeable board members. Not only will you be doing something positive for your local community, but you'll also open yourself up to new connections and prospective clients.