The hardest part of building a mobile-first startup is distribution, or, simply, reaching out to your target customers for sustainable traction. Aside from a bad idea, most apps fail because developers were not able to market their products.

A lot of entrepreneurs make the mistake of only focusing on acquiring users, rather than engaging current ones that are actively using the app. Everyone wants viral effect, but no one works towards it.

The only thing that causes a viral effect is word of mouth, which is best spread by those that have used your product.

Related: To Boost App Downloads, Boost Traditional Marketing Efforts

Do you see where I’m going with this? Your best marketing channel is your existing customer. What have you done until now that makes them bring on other users to your app?

Let’s look at a few strategies that will get your users to market your mobile app for you.

1. Aid referrals. Dropbox ran an extensive campaign during which you could share the service on Facebook and Twitter for additional space. Referrals increased Dropbox signups by 60 percent. Dropbox makes it really easy for users to tell someone else about the product. For example, when one person who has Dropbox refers another, they both get a 500 MB increase, once the user signs up.

PayPal paid cash -- $10 to each new customer and $10 to the customer who referred them. It worked brilliantly for them.

Find a way to incentivize your users to spread the word.

2. Incentives that drive marketing for better product experience. Another example from Dropbox illustrates this point better. With each follow on Twitter or like on Facebook, Dropbox gives the user a 125 MB increase. It’s no longer fashionable to just put a "Like us on Facebook" or "Follow us on Twitter." You need to give users a reason.

3. Vanity content that inspires sharing. While Runkeeper is an app that tracks and records your runs each day, providing data on distance covered, calories burned, etc., it enables users to share this data on their social networks each time they work out.

Related: Laugh All You Want: Sales of Kim Kardashian's Much-Mocked App Are Astronomic

Users proudly enable this option of sharing on their social networks because it makes them look good among their friends and peers. It’s vanity. People love to show off! Imagine the publicity the app gets when users post their stats on their Facebook profile.

4. Shareable content via email. Create a landing page for your app that collects emails of your users and potential users. The bait could be an ebook or premium content that is not available otherwise in public domain.

Just look at Neil Patel’s blog, QuickSprout, where he offers a free tool to learn the number-one reason why people aren’t getting enough targeted traffic to their website. The content you create has to be so compelling that your users want to share it with their own network!

5. Outstanding customer service. Everyone talks about customer service, but no one understands the gravity and importance of it, or even the right way to do it. Eighty percent of companies said they believe that they deliver superior customer service, but only 8 percent of customers think these same companies deliver, according to HelpScout.

If you were to take a cue on what’s the best way to do customer service, take it from the team at Buffer.

“Customer support is the very rare opportunity to connect to your customers on an emotional level," said Buffer co-founder Leo Wildrich. "You can’t do that in any other way.”

Can you carry this attitude while building your product and servicing your customers? Happy customers often spread the word.

These are some of the tried-and-tested strategies and can certainly work for your mobile app in one way or the other. If these did help you, I would love to hear from you in comments section below.

Related: Synergize Marketing Efforts to Gain New Customers