Continue developing and refining your main characters - try to have them nailed down.
Make clay models of your main characters
Build a model set to place your characters in for photography purposes
Draw a line art version of your characters - make 5 copies to use for color studies
Choose 2 to 5 books that have the color palette you have in mind for your book (you can use the color palettes in these books for your character color studies)
Keep working on and tightening up your storyboards - move to the larger storyboard template
Read Perry Nodelman, Chapter 3