A few approaches:
One is to just sit down to sketch and like... just start with what you normally do. Then for the 2nd try, do something totally different. If you drew a tall guy with short hair, maybe do a short girl with long hair next. Then try something different for the 3rd one: maybe if both of these characters were thin, try drawing a heavier character, or one with more toned muscles. Then maybe the next one you try a different jaw shape, and the next one different nose and eyes, and so on. Experiment with shapes, go bonkers! Same can go for animal characters- if you start with one dog or fox-like character, maybe try a cat next, then a bird, then a lizard, then a fish, and so on. Or sticking within one species you can go through similar experiements: how do the different genders look? What is the difference between a buff person in this species vs. a thin person, and so on. Same with robots and whatever really.
Once you do some practice like this, then when you sit down to sketch ideas for new characters you can have a larger pool of ideas and shape language to choose from. It's easier to dedicate time to this sort of practice than trying to generate it out of thin air while simultaneously trying to design a character.
The second thought is basically the same as the above, but while making heavy use of reference. Take time just sketching different hair styles, noses, mouths, clothes, animals, etc. Try to take note of how each is different, how to structure different ones, and how to translate those elements to your style. Or different species of similar animals to see how they differ. & etc.