Make sure you keep Lisa's ears close to her eyes and nose, but also make sure the nose line is perfect and overlapping the eye. Also, don't forget to keep a streamlined curve to the face and keep the eyes round. This is important! Lisa Simpson, an iconic cartoon character as there is, is instantly recognisable anywhere because of the details put into drawing her. These instructions, which first appeared in Simpsons Illustrated years ago, reveals how detailed a cartoonist needs to be.
I just love this animators style guide for Lisa. I love how there are clear no no's in drawing Lisa and how there are things that are acceptable. And it's true, you can see all those foreign faces of Lisa above and feel a little weird because you've never seen a Simpson drawn like that. Things like how Lisa should never move her upper teeth up and down but instead have their bottom teeth moved up and down. All this effort pays in developing an iconic character.
This page, which was pointed to recently by @MakingOfs, makes me curious about the rules of every other character.