The mathematical trick is a different story. I'm still trying to make these so called "self working" tricks look magical. Sometimes, the procedural nature of these tricks gives it away. I think the ideal mathematical trick is one where the procedure doesn't seem to affect anything. If you can add any insights about this, I'm ready to listen.
I'm sorry to steer of the subject, but maybe I can help here.
I had similar problem about a week ago. I was performing at a night club, for some girls (friends of a friend), and one of them was very smart (in general, not for magic). And she was also very analytical, and tried to figure stuff out. One of the reasons that she may wanted to do this is because she wanted attention. There were 7 girls, and it is really hard to involve them all at the same time. And guess what sent her to the floor? Yep, the simple mathematical trick. After the performance she told me "All this things with cards, they look great, but everyone can learn that, I mean it's all just sleight of hand. But how the hell did you do that thing with numbers? Did you read the reaction of her face, and that is how you knew the numbers? I just can't understand how....".
Given the right presentation, you can fool the most, with the self working trick. And those tricks are the best ones for practicing presentation, because you don't have to worry about sleights.
Oh, btw, here is my presentation of a said mathematical trick. Hope it helps.
http://forums.theory11.com/showthread.php?t=22579
Cheers