One of my favorite techniques for getting into the head of my characters is, interviewing them. This relies heavily on rapid writing, so be sure you fully understand that concept first. If you don't, you can read my article "Rapid Writing" on my den. Interviewing characters is exactly how it sounds. You sit down with your characters and interview them, just like you would if you were a reporter.
To start, take out a blank piece of paper or start and new word file. Decide where you are going to meet them; in their house, coffee shop, or somewhere else. Now rapid write about it. Go in and meet them. Say hi. Record what he looks like, how he acts etc. Now the fun part. Ask questions. Ask your characters about themselves, and about the story you're trying to tell. Get them to tell you their story, in their own words. Let them tell you what happened and how it affected them. You may gain new insights into your story. The important part is to not think or judge what you're writing - just rapid write. Don't worry, you're mind will fill in the blanks. Write until you think you know enough about your characters or stories. If you run out of questions, write about there being a lull of silence in the interview. If you get past the 'wall' something else will come. Just don't stop writing until you feel you have enough new insights to go back to writing.
This technique also works If you get stuck in the middle of writing or plotting your story. You can go back and talk to your character about the section you're stuck on. Let him/her tell you what happened.
To help with this exercise, I have included some sample question that you can ask. You can use these, but also allow space for your mind to come up with questions on it's own.
1. Tell me about the story I'm writing. In your words, what happened?
2. How did the events affect you?
3. Are there any details or events in the story that I've forgotten?
4. Describe yourself. What are your hobbies, dreams, hopes etc?
5. What was the most important thing that happened to you during the events I'm writing about? How did that change you?