There was recently a free video over at Snakeymagic.com about adding drama to an effect. The principles there are actually quite good. Seal the prediction up, give it to someone to hold. Put money on the line (this one is only really great if you can give them the money after the effect anyway, but that's just my opinion).
I came up with a card prediction, where there is a card from a different deck in a sealed envelope that is given to a spectator to hold, and another spectator removes the deck from the box, shuffles as much as they want, then deals out the cards face down onto the table. They stop when ever they want. They are given free choice to stop or continue or even take some back. Once they have stopped, the spectator opens the prediction and shows everyone. The other spectator turns over the card they stopped at, and of course it's a perfect match.
Everything is examinable before and after, and I never touch the deck at all the entire time.
What works well is the sealed envelope, and me emphasizing that I have not touched the deck of cards. To the spectator the only thing I have done is sealed a card in an envelope. They do the rest themselves.
A big part is in the presentation. A few magicians I have seen use the Master Prediction Box which is a large metal box with big locks that is suspended before the show begins. Near the end it is opened to reveal some details about the show or whatever the prediction is. Now the effect is heightened because of the presentation.
1) The box is suspended before the show starts, so there is not way the magician can tamper with it
2) It is a large metal bog with large locks on it. This makes people feel that whatever is in there is very secure and that it would therefore be impossible to tamper with. This is similar principle to a magician being tied up in a hundred feet of chains with big locks instead of ten feet of rope. The rope is more difficult to escape from, but the chains and locks give the audience the illusion of difficulty, when in fact it is easier.
3) The information inside is usually of a personal nature to the audience, thus increasing the impossibility of the magician knowing before hand.
I hope that will help you with your effect!
justin