Hello! First, thank you for saying such a nice things about my sims! It truly means a lot to me.
What can I say... I ALWAYS use inspiration from real faces. For me, eyes and brows are things that define face the most. Having a reference helps achieve a little "life-like" effect. If you manage to properly recreate the eye area, you can actual see emotion in your sim - melancholy, happinness, boredom. It depends on person/picture(s) you base your character on. Download as much sliders as you can and don't be afraid to play with each one! Sometimes that one slider that you've always avoided can help achieve the look you've been aiming for.
I'm unable to make sims with maxis-match skins. I always need to have some realistic details that define the face. Eyebags and eyelids are most important.
Hairstyles are very important part of the sim. While creating my character, I like to imagine what kind of hair should fit them. I have two main styles - straight, more thin or puffy, thick hair. In my opinion, it really DOES help achieve the perfect look. For example I can't imagine Pola with very fluffy hair. On the other hand, Maree will always looks best with more volumish hair.
Well, I guess that's it. I have no idea if it's helpful in any way though 😅