Ok to answer what you wanted to know about being convincing,
don't make outrageous claims if you have to make your explanations sound simple and acceptable, if they wrote a hidden word down and you some how guess it do not say
'YOUR EYES GIVE THE WORD AWAY'
But something like this,
'Startling I know, it's unbelievable but the technique is simple, it is an old principle used by charlatan psychics to make it look as if they guessed the name of a person that the client wanted them to contact.
The way it works is very simple, I ask you to repeat the word just in your mind, over and over then without knowing you move your lips very slightly and give away the name, once you have the first and last letters its not I'm sure you will agree hard to deduce what the middle few letters are.
I am totally honest about my dishonesty and I'm not psychic in the slightest and did not want you leaving thinking any otherwise thank you for helping me out'
NOW LET'S LOOK AT HOW THE SUBJECT WILL SEE IT.
We know that all we did is got an impression from the 'pad', but the subject has no reason to think about that as you gave her a fake explanation as to how it worked.
You said you were honest about your dishonesty (which is another lie) and you didn't want her leaving thinking you was psychic (putting the blame onto someone else) this is more reason for her to believe you as you are pointing out that people use this method (movement in the lips) to pretend to be something else and finally the most important part of all you gave her an explanation where she feels shocked that she gave it away in such a simple manner and wont question the credibility of the bull you just fed her.
credibility is not based on 'making' things believable, it's about pointing out things that seem so obvious upon reflection they must be believable.
Last thoughts - giving massive reasons instead of simple ones is a big NO! as you have to make excuses for your excuses
hope this helped