I worked on an open source application, and some people wanted to use parts of it as a library in their commercial applications. So I started a consultancy due to that demand. I still had a regular job at the same time though, so there was never a need to gather enough clients to make a living out of the consultancy job.

Things I learned:

- Get an accountant ASAP, even if the income is small. Just the peace of mind that my taxes were being filed correctly was worth the cost.

- You don't need a perfect solution from the start, you are working with your client towards something they can use.

- You need to stay on top of things and communicate regularly, even if your client doesn't.

- Almost all clients wanted me to either come work for them or sell all (rights of) my work to them. This is understandable from their side, but if you want to stay independent you need to set some boundaries.