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Must reads:
Katy Coope (Scholastic), How to Draw Manga,
(Ikari Studio), Draw Erotic Manga,
(Studio Imagiks), The Mechanics of Anime and Manga
Avoid the following at all costs:
Andrew Loomis, ALL HIS BOOKS
Betty Edwards, Drawing on the Left Side of the Brain.
You can draw from reference, but only look at anime and manga. Avoid photography or fashion magazines, and don't hire a model or go to live drawing classes. Nudity in real life absolutely forbidden, but feel free to draw from your favorite books.
When drawing, it is ideal to put on music with lots of vocals. Try something that uses synths and sampling heavily, not music that is performed. My favorites are Lady Gaga, Vocaloids and Konami Bemani songs. Modern R&B is also highly effective in improving your skills. Talking to yourself about your wishes while drawing also works.