If I have a bad art day, either that I just can't draw anything properly that day or that I'm in the right mindset to work on my comic/other project.
I always walk away from it. I'll go for a walk outside if it's nice out, watch a movie, listen to some music, clean. Really, anything else will do. I'll come back to my projects in the evening or a few hours later and try again. Sometimes, all you need is to clear your head or relax.
Two weeks ago, I had a bad art week. That was torture! I decided to write instead and plan my future chapter bit by bit. I also kept busy with other things and relaxed. I do this on my personal time.
But, if I'm having a bad art day at work, I usually go online and look at random designs. Usually something triggers me and I end up being able to design whatever assignment they gave me. If that still won't work (extremely rare cases), I go on break and have a snack. It sounds stupid I know, but I'm on deadline, so I have to push through somehow. I just know, stepping away from it, usually helps solve the issue.