I seem to mess up phone interviews as well. I would say that the best way is to be kind of meek to avoid sounding like a bumbling idiot or a pompous know-it-all ass. It's all too easy to mistake enthusiasm as ditzy or pompous or 'says-10-things-but-only-2-of-those-were-necessary-and-important', so I just usually say only what is important and always with a serious voice that doesn't sound too smooth-talking-MBA-ish. I also try to be humble and play the role of the meek student so that it is clear that I do not think I am king of the world, and the meek part helps show that I wouldn't put up a fight in following directions, I would just follow them and want to learn the correct way of doing things (the correct way being the boss's way, of course). I find that people generally like this attitude because it is not threatening and it is also flattering to have a smart, quiet person who is obedient to you.
As for the technical parts...well, you have to know your stuff. But it's important that you answer with this kind of attitude as above, not the "yeah of course I know that, the answer is St exp miu minus half sigma squared ... "
This is just my opinion. Maybe there is a better way but this is the best one I thought of.