You show a sealed window envelope inside of which can be seen a piece of paper containing your prediction. The envelope is handed to someone of importance who will be at your show. It could be a committee chairman, police officer or local celebrity. he or she signs and dates the envelope so that they will recognise it as the same one later. They also scribble across the flap so that any tampering would be obvious, should the envelope be opened prematurely. A strip of Sellotape is then placed across the scribble to protect it.
The window envelope is then dropped into a Jiffy bag which is sealed and handed to the celebrity for safekeeping.
On the night of the show the celebrity brings along the Jiffy bag. It is opened and the window envelope removed. It is the same envelope, still sealed and with the celebrity's scribbles intact. The prediction can be seen through the window in the envelope. Held at fingertips, the envelope is sliced open with a paper knife and the prediction is removed and handed to the celebrity. Incredibly, it contains an account of that day's newspaper headlines.
It's just about as clean as it gets.