Writing the other gender

At Oak Knoll Middle School outside of Richmond, VA they have a wonderful reading program. In it, students and their parents choose a book to read together and then they meet weekly to discuss it. I was fortunate enough to have both the boy and girls pick my book, The Hidden Sun. It was the first time both genders had picked the same book. After they completed it, I visited the school and gave a presentation. It was a blast.
The experience brought me to ponder on why The Hidden Sun appeals to boys and girls. It wasn’t really my intention when the book was written. I’ve come to the conclusion that it was a combination of things.
First, the story is written from several different points of view. In each “scene”, we are in the head of someone. Sometimes it is a male character, sometimes it is a female character. In that regard, I believe both genders who read it had someone to relate to.
Second, there are elements in the book that appeal to both genders. There are action sequences and competitions that the boys enjoyed. Then there are the romantic elements that the girls liked.
I’m currently writing my fourth book. It’s the third book in the Bariwon series. The main character is female. While I’ve been writing it, I’ve read several books from other authors. Often, they are written from one person’s point of view.
In one particular book I read, the main character was a teenage young woman. I’ll admit I struggled a little through the book because I had a hard time relating to her. Being the father of four daughters, as well as being married for twenty years, I’ve learned that men and women simply do not think the same way.
If that’s the case, how does a male writer create a believable female character and vice-versa? I’ve come to only one conclusion: through observation. The female characters I create are composites of various people I’ve known over the years. Even then, I’m sure there is a lot I’m missing.

I once attended a bachelor party (it was an LDS party, so it was very clean) and one of the gifts given to the groom-to-be was a fairly thick book called Everything Men Know About Women. When the receiver of the gift flipped through the book, we saw that all the pages were blank.