what you've got is a good start, something that i think may help is getting familiar with what len mentioned about turnarounds or working with the head/face from various angles
i know not everyone can do the picturing stuff in their head thing but understanding a characters head or face as a 3d object than just a 2d image can help a lot. my method usually falls to basic shapes so squares, circles, triangles or their 3d counterparts spheres, cubes, cones and pyramids
so when applying it to a face you can start with shapes and build on them like these:
as for turnarounds it's kind of the same thing taking them and rotating them
as for face shapes that'll boil down to style preference or the kind of character you're looking to draw or design since that's something you can use to create visual variety in a cast lineup or show off certain personalities and what not