Here's a guide to manga heads:
On your picture: the eyes shouldn't slant down, the ears should be lower. Both ears and both eyes are different sizes and one's placed higher than the other. The neck should be straighter and thicker at the top. The top of the hair should be higher off the top of the head. The nose and mouth could stand to be a little lower.
The best advice I can give you is to do a hair and head study. Draw from references and study anatomy really well. It's the best way to improve!