You will want to think about Janie's clothes and appearance as part of the novel's overall theme of a woman's search for self-identity and independence. I will talk about Janie's hair in this answer and that should hopefully give you a few ideas to think more widely about her appearance in the overall context of the novel.
Janie's hair and in particular her way of wearing her hair acts as a symbol of her strength, determination and power, but also of her unconventional identity. This can be argued on the basis of two points. Firstly, it shows that she is not going to be a woman who will "fit in" and change her appearance to suit cultural norms. We see at the beginning of the novel that the town thinks it unsuitable for Janie to wear her hair down at her age. Her refusal to change her appearance based on this disapproval shows her strength of spirit and rebellious nature.
Secondly, her hair is straight, and not curly, and thus functions as a symbol of whiteness. Mrs. Turner in the novel has a very high opinion indeed of Janie because of her "white" hair and her other "white" characteristics. Her hair therefore contributes to her ability to disrupt the status quo in power relationships throughout the novel (male vs female, white vs black).
Another point you might want to consider is the way Janie's hair is described and explained. Some critics argue that in its braided form it is described in phallic terms, which further serves to blur gender boundaries and which some male characters find threatening.