I believe the key is eye contact. Looking someone directly in the eye while talking (even more important when listening) is such a powerful action. It's amazing how rarely we really look someone in the eye as opposed to just looking at their face. I've been focusing on doing this when out in public, particularly in those passing encounters, say, with a clerk at a store or when saying a quick hello to a neighbour and there seems to be a significant reaction from the other party- assuming they actually look up to notice
As for charisma, I believe charismatic people exude confidence and positive energy. I don't think you really have to be a fully confident person to appear charismatic, but carrying affirmations of confidence when in contact with others helps a lot. Smiling is another key and I don't think it matters how you smile as long as it's natural and appropriate to the circumstances. Charismatic people also seem to be inclined to touch others in a non-intimate, respectful way.
I am a firm believer that this stuff comes from inside. If you feel it, others will respond. The one thing I would be wary of is focusing on the behaviour so much that your behaviour becomes affected (and I mean that in the sense of "fake"). We all know people who come off that way and what a turnoff it is.