Here's what I think:
1. How can I have more "magnetic" body language? Learn to be self-confident and comfortable with who you are and the body language will follow.
2. What exactly is an "open posture"? Where am I supposed to put my
hands/arms when standing and sitting? Again when you are self-confident and comfortable with who you are this comes naturally as well.
3. What makes someone charismatic? How can I be more charismatic? As Jenny said charismatic people take an interest in others. They are good listeners and maintain good eye contact.
4. How often should I smile? Should this be a show-the-teeth smile, or a curl the sides of my lips up smile? You shouldn't smile unnaturally or just for the sake of smiling. People will think you're not all there. You can smile to greet or acknowledge someone or when you are amused by something. If you find something funny by all means laugh. It is very attractive to people.
5. How can I have a more positive/uplifting personality? By being a positive, friendly person. It will come through.
Whatever you do be yourself. Be natural and down to earth. If being yourself is difficult because you feel awkward or self-conscious then practice affirmations
on how you want to act. It's amazing how helpful they are.
Work on yourself to be the person you want to be.