I usually switch mediums. It always helps to jog me out of a rut.
The worst is when I spend a lot of time on a sketch, sometimes I know it is wrong, but I just keep investing time thinking I can fix it. Then I step back from it and realize it sucks and punish myself for wasting an hour on garbage.
My best sketches tend to be when I’m thinking about something else entirely, and it is just a quick little flow, effortless. It can be hard to get into the state of mind. Music helps of course, as does the setting, I almost never ever sketch at my desk. At my first job we all sketched at a conference table in the middle of the studio and I got accustomed to that. It feels more causal.
My favorite time to sketch is early. I’m usually the first one in the studio and sometimes when I get in, before I even take my laptop out of my bag, I’ll put some music on loud (we have a lot of audio equipment) and pound out a sketch or two.