Making passive income is a great thing to be doing as a designer. It allows for potential exposure and future clients. There are many forms of passive income some of the one that I feel are the Most effective are creating things and selling assets like website templates, textures, icons, etc. there are a lot of ways to make money while you sleep as a designer. Opening up a small web-store with a third party site like society6 for example. It really comes down to figuring out what you have fun making the most and turning that into your passive income.