I talk to my clients a lot about diversifying their income streams. It's like not putting all your eggs in one basket. And when you are self employed (or investing) it's pretty important.
Yes, I believe that if you are a designer you can be successful at doing JUST that, ONLY that. But, in the interest of your mental health (ie sanity), I strongly suggest you have some different revenue streams that you can put your feet up on during the leaner months.
For those lucky self employed, married folks with a spouse who is employed by someone OUTSIDE the family (not me, it is true) they have a bit of a buffer from the ebbs and flows of income.
I have a bunch of different income streams I rely upon. Not only does that make me feel more secure financially, but it makes me more interested in what I am doing. I can switch gears when I am stuck.
How are you financially flexible? What can you do to be MORE SO?