Immerse your reader in a world that is familiar, but one that also has unique features. This was done very well in the "Harry Potter" books by J.K. Rowling. Kids from all cultures understand love triangles and struggles between good and evil. The world of the Hogwarts school of witchcraft and wizardry is an enchanted place that children (and adults) want to understand and even visit.
Ground your characters in reality. While it's easy to write about children that are childish, it's much better to give them special characteristics that will increase the meaning of the drama. Children often feel hurt, vulnerable and alienated. Depicting a character with vulnerability and pain--as well as wide-eyed wonder--makes all the difference. The character of Ender Wiggin in Orson Scott Card's "Ender's Game" is told he is the savior of humanity, but he is still a young man trying to understand himself and his place in life.
Understand the difference between making use of archetypes and doing the same thing as everyone else. Sure, all great stories have many of the same elements. A protagonist who has big challenges in front of her; a mentor character who may not make it to the end of the novel; the acquisition of new powers--these can be prominent in your plot, but do your best to write your own story instead of creating a plot your audience has read before.
Remember that young adult novels don't have to appeal only to boys or only to girls. The best books will appeal to both. While there are certainly differences between males and females, everyone appreciates a good story.
Refrain from patronizing your audience. Just because some young adults may not understand a word or concept you wish to use doesn't mean that they aren't smart enough to figure it out. Young adults love challenges, and they like to be treated with respect, knowing that the author is on a journey with them, not just telling them what to see in their minds' eyes as they read.