One exercise I do sometimes is breaking down objects into planes, then adding shadows/light to the right planes depending on their angle relative to the light source I'm envisioning. I think it's very useful to get a deeper understanding of an object's 3-dimensional form, and I noticeably improved at drawing faces after doing this just a few times.
With these hands I'm not strictly doing the 'plane thing', but I'm still thinking of them as 3-d objects and trying to simplify the forms, light and shadow areas in a way that seems appropriate to me. I still often can't draw good hands without reference but I definitely think exercises like this have helped.
The most important thing IMO with these practices is really trying to understand the form of what you're drawing and not falling into a rote copying mindset where you just imitate the angles and shapes. The better you understand the form, the more easily and convincingly you can simplify or exaggerate it.