Remember my post a couple weeks back on finding your character's core? I want to expound on that a bit because one way to ensure a strong character arc is to chip away at that core belief and alter it over time until at the end of your story, it has changed in a fundamental way.
It isn't easy to change someone's core belief, so it's going to take some work on your part. But that's what will make the shift in character so interesting and realistic.
Let's use HUNGER GAMES as an example, since most of you have read it. Katniss' core belief at the beginning is something like: I'm the only one I can depend on.
Katniss had only herself to care for her family when her father died. Without her they would have all died. She learned from her mother's desertion (emotional withdrawal) that she couldn't depend on others. As a consequence she didn't let others in easily. In fact until the Hunger Games, she only let in Gale and Prim. Throughout the games she assumes other's actions are the result of ulterior motives. That is until Rue. And eventually Peeta's motivations cannot be explained away.
By the end of the book I'd say her core belief is more like: I can protect myself and make a difference for others.
Have you identified your character's core? Do you know how it changes throughout your story?