There are several ways to do this based on your interests. I like “passive income” so that I have a product out there bringing in an income for me without a “continuing need” for me to work with it. For example, I have authored a book and a continuing education course both of which have the potential for providing a continuous income stream once they have been created. Also, since I like to write, writing articles for paid publication allows me to achieve additional income without adding to my training time. These are just a couple of ways in which a trainer can bring in additional income without increasing their training time.
I hope this provides you with a couple of ideas.