I’m not trying to reveal too much, but I have a question about Mnemonica. Suppose that someone picks the JH as their card, and you ask them to pick a number between 1 and 25, and they pick 2, what out would you use in a situation like that?
I would take the deck out of the box and pass or cut to the 18th card. This would put JH at the 2nd position. Or if you buy the book you can Cut to a card and spell to it. Many different options just be creative.
There are good methods for doing this in the book Mnemonica.
An easy method is to turn to another spectator and ask "since there are more than 25 cards in a deck, can you pick a another number between 1 and 20?" Add the two numbers together. Let's say they pick 16. You tell them, "What I want you to do is deal off cards one at a time and count, like this...1 [deal one card], two [deal second card]." Return the cards to the deck and hand it to the spectator.
Another method would be to do a three-part cut of the deck to the table cutting at the Nine of Hearts (put 1st pile with bottom card 9H down) and around the Jack of Clubs (put second pile with bottom card JC down to the left of the first pile) with the last pile (with bottom card 9D) going to the right of the first pile.with piles having 9H, JC and 9D at bottom. Pick up the left pile (JC on bottom) put it on the right pile (9D on bottom), put both piles on top of the middle pile (9H on bottom) and then hand the deck to spectator.
If they pick the JH I wouldn't give them the option to pick a low number. I would guide them to a higher number, something like: 'I want you to pick a number between 1 and 25, but high, very high. Difficult one.' Your spectator doesn't know why, so he'll 'choose' a high number.
Let's say they pick 23, show how they should deal cards (face up to the table, hand,...) and use 3 cards from bottom for this. After dealing these 3 cards, add them to top. Depending how you took those cards, cards 50, 51 & 52 are not in the correct order. Fix this after the effect.
If they take 18, show how to deal with 2 cards from top, and add to bottom.
I second RealityOne's tip, Juan explains a lot of this in Mnemonica.