The line of action and body language help me a lot when I have trouble on getting an expression right!
Use the facial features as guidelines. Look in the mirror and try different expressions! There’s a difference in face shape for when you smile and frown. I mostly change the cheeks, chin and eyebrows in getting a squash and stretch feel.
Just remember to think through what your character is doing 8) How far do you want to push someone screaming? someone smiling? someone talking, crying, getting angry, looking confused or just neutral! It gives elasticity to faces which opens up possibilities for expressions! Experiment and you can even push it further with neck length, hair, etc! Hope it helps!