1. Word of mouth referrals. This is the old school way to get recurring revenue. But I've seen a lot of freelancers fail at this. Specifically, delivering the client exactly what they envision AND positioning the client to be successful. If they aren't successful, then they're going to be out of business soon, and that means you're going to have one less word of mouth referral out there.

2. Support contracts. If you've done a good job at 1, then there's a good chance that you're the client's only line of support for their project. Just because you've delivered all the code and launched the app doesn't mean the work is done. What happens if their users have questions or feature requests? What happens if their site goes down? By offering support contracts you can continue to invest in a client's long term success, keep that referral hot, and start building recurring revenue for your business by having a support contract that renews annually or quarterly.

3. Productized consulting. As a freelancer you learn a ton of skills on the job. Sometimes these skills are applicable to other clients. If so, you should package this offering. 37signals did this a long time ago by offering to redesign a single page on a site for a flat fee. I have interviews with people who are doing this kind of thing with Stripe integrations. The other side of this coin is that you have to position yourself as an authority on the package you are offering. By doing so you further diversify your recurring revenue by having leads generated based on your expertise.

4. Product sales. This is kind of the 'unicorn' of the bunch. If you are building an application for a client that has no interest in selling or owning it (i.e. research departments, internal tools for large organizations, etc) you can take ownership of the product and sell it to yourself. I've seen this happen a handful of times and it essentially multiplies your revenue because it produces work in 1, 2, and 3.

Hope this helps.