I think the most important thing that has saved me from art block so far is my process. After years of being a full-time artist (one of which I did client work), I developed a bullet-proof way of walking through an illustration - from start to finish. I can always rely on it, even if I don't have any ideas before I sit down. But as soon as I start going through the motions, things fall into place.
Inspiration exists, but it has to find you working.
― Pablo Picasso
I described my process in detail in this interview with TOR. Of course, this is MINE. Yours might look entirely different and it's up to you to figure it out.