Hey @lunaticmoon ! It depends but something that really helped me to add a bit of diversity was a combination of a few things but mostly volume practice.
It's like understanding that you have basic volumes (sphere, cube etc...) and all of them can be bent and modified. After what I applied it to a subject and played with its volumes/proportions. For a head it can be the nose, longer, larger, shorter, arched... or the head shape more curvy, sharper....
And by testing and doing lots of mistakes, you manage to see how far you can go haha (that's why I talk a lot about thumbnailing, it allows you to test things)
OH And last thing : If you get stuck with a generic head, ask yourself what would be the opposite ? For exemple if your head is spherical, what would it be if it was based on a cubic shape ? or if your nose is short, try to make it bigger.
(My exercise was spiders as I like spiders and they are based on cylinders and spheres a lot, so easier than a human body but the key is volume I think)
Does that make sense ?
I hope it helps ! Cheers !
I'll be tryna do lots of exercises as much as i can soon!mhmm that makes tonnes more sense @alcy !