Well, I got bored of good looking sims from the very beginning so I decided to make 90% of them look average or even ugly (but in a realistic way). I make the space between the eyes either wider or less wide, move the mouth and nose higher or lower on the face, modify the lips, make the faces long or flat, etc. it's quite fun and I end up playing with unique looking sims. Sometimes I even give NPCs make over to make them look special and when they have children autonomously (thanks to MC Command Center) I recognize them at the first sight without having seen them ever before and I'm like "They definitely have their father's nose!"
