Don't forget attractiveness. "Atractive" people (physical beauty) are usually more charismatic and people tend to trust them more than less attractive people. Now by no means does that mean less attractive people can't be charismatic, actually, charisma is really pretty much attractiveness in a sense.
I know you can become more charismatic, I've done it. Now, maybe it's a natural thing you have, but you can learn to improve what you've got. A lot of people can make themselves look sooo much better if they put the effort and did the right things. Again, I've done that too with great results.
But I do have to say, it seems that some people just have that natural charisma that lets them get away with anything.