User onboarding is already a continuous process itself. The aim is to assist the users repeatedly to find new value within your offering. A successful onboarding process helps users advance further by revealing what is more beneficial for them.
Promoting new features for an existing user is a great example, herewith you will be able to encourage them to achieve more with your product.
Some of the good user onboarding or CRM tools help you establish action based processes so that you can keep contacting the user at the right place and time throughout the user lifecycle.
Onboarding is not a task that can be accomplished just with a good interface design or an instructive product walkthrough. You need to be there for your users each time they are in need and put forward the next cool feature to increase the value of your product.