Interesting topic to read about.
Most time when I tighten up is because I try too much and force myself to keep on sketching. The first thing I try is to scribble a bit on a new paper, it is a bit like the practice drawing. When that doesn’t help I put my mind on something else by doing something else as IDAL mentioned.
Sometimes I try to get inspired by the work of others on websites and in books. This can also work negative for me as I do think all my doodles don’t look good in comparison to the work I have seen.
Finally I just quit and start working on it another day 
By switching mediums do you mean from paper to digital or more from a pencil to a felt-tip pen?