What does Dorothy fail to prepare for her trips? Why does Dorothy not prepare for any of her trips?

In The Wonderful Wizard of Oz, Dorothy ends up taking a number of trips, and she prepares very little for any of them.  Dorothy doesn't prepare for her initial trip to Oz because the trip itself is completely unexpected.  At the start of the story, Dorothy and her house are swept up in the cyclone and taken to Oz.  After crash landing, killing the Wicked Witch of the East in the process, Dorothy meets the munchkins and the Witch of the North.  She is told about Oz, and given the silver shoes and a protective kiss from the Witch of the North.  Beyond that, the only thing she does prior to departure is change into her only other dress, which is "gingham, with checks of white and blue."  This is very fortuitous, as white and blue are colors that are highly revered in Munchkinland.  As Boq tells her shortly after,

"Blue is the color of the Munchkins, and white is the witch color; so we know you are a friendly witch."

That is all that Dorothy does in preparation for her second trip.  For her third trip, from the Emerald City to the castle of the Wicked Witch of the West, Dorothy prepares very little.  Before she and her companions leave the Emerald City, there is a sort of "training montage" in which they all start to prepare in their own ways, at which time a green girl "filled Dorothy's basket with good things to eat."  Beyond that, she "put new paint on [the Scarecrow's] eyes that he might see better."  That's it.

Finally, after the Wicked Witch of the West has melted, Dorothy and company begin the return journey to Oz.  Here the only thing that Dorothy takes is the diamond-studded bracelet given to her by the Winkies and the Golden Cap that allows the wearer to summon the winged monkeys. 

For the final lengthy journey, to see Glinda the Good Witch of the South, Dorothy prepares absolutely nothing.  Upon reaching Glinda, Dorothy is informed that her journey home will occur if she simply knocks the heels of her silver shoes together three times and commands the shoes to take her where she wants to go.  After saying goodbye, Dorothy does this and ends up back in Kansas, losing the shoes in the process.

Throughout the book, Dorothy shows very little preparation whatsoever.  This is in part because the book is for children, and lengthy, detailed sequences can become boring for kids, but also because L. Frank Baum was setting out to create a modern-day fairy tale.  He wanted to tell a light tale "in which the wonderment and joy are retained and the heart-aches and nightmares are left out."  This also contributes to Dorothy's lack of preparation, and the lack of any repercussions that might come of being unprepared.  Baum tells the story of adventure rather than of the build up to it.