Staying in Character Part 2: Writing in First Person

For some writers (me, for example) it's difficult to have a different voice when writing in first-person.

One of my stories is from a boy's point of view, the other is from a girl's and they sound, if not exactly the same, very similar.
They both sound like me.

So I've been trying lately to create a character completely different from myself and then slip into their skin.

Beforehand, I started writing in first-person before even figuring out who they were. That only resulted in a duplicate of myself every time.
You are looking through their eyes.

What do they think about?
Are they slow and hesitant to make decisions or headstrong and reckless?
Are they optimistic or cynical?
How do they see the world?