You shuffle a deck of cards, and invite a spectator to play a simple game - highest card wins!
You cut the deck and end up with the 9H. You feel so confident about it, you put a £20 note on the table. Highest card takes it.
The spectator cuts, and turns their card over... it's the Jack of Spades! They win!
You hand them the money, and suggest they unfold it. Like in all things that seem too good to be true, there's terms and conditions...
The banknote says... 'UNLESS YOU CUT THE JACK OF SPADES'.
'Thanks for playing!'
This effect is one I've played with for a LONG time, and up until recently, my preferred method was a one-way deck. I had two indifferent cards, so I could flash one as I removed the pack from the box, and the other one was a breather crimp in about the centre. I cut to that, and tabled it. The spectator then could only cut to one of the many force cards. With the deck tabled face up with one indifferent card showing, and my losing card face up, and the spectators card, there was just enough different cards showing to prevent awkward questions.
However, the thing is, the trick is so direct and over so quickly, that it's NEVER going to be the only trick you do in a situation. So here's how I do it now, with a borrowed, shuffled deck.
Beforehand, write 'UNLESS YOU CUT THE JACK OF SPADES' on a banknote (you can first cover the note with sticky tape, and write on that, which is then removable) and fold it in half, writing side hidden. Have this in an accessible pocket or orifice.
Show the deck of cards, cut or cull the Jack of Spades to the top, and false shuffle, retaining the card in position. Explain that people never want to play cards with you, so to get a game, you have to put up the money for a bet, and your spectator doesn't.
Explain that you are going to play a very simple game that will just use one card each, and casually spread the deck face up as you do so. Cut the pack anywhere at a card with a lower value than the Jack of Spades, and table the halves face up, your spectator with the Jack of Spades portion, and yours with the X card (say the 9H).
Bring out the note, and place it, still folded, as you explain that it's up for grabs.
Demonstrate that you want the spectator to cut their packet by cutting yours, and then pick up the lower section and set it on top of the upper portion at right angles (or close enough). This is, obviously, the Cross Cut Force, but you are doing it at the same time as your spectator, and rather strangely forcing a card on yourself.
Justify the 'marking' and add some time misdirection by picking up the note and snapping it open (keeping the writing towards yourself) and commenting that the note doesn't only have monetary value, but also sentimental - it was the first money you ever earned by doing magic... it's two weeks old.
Table the note again, and lift off the top portion of your packet with one hand, and with the other, remove the top card of the bottom packet and slap it face up on the table. It will be the Nine of Hearts.
I keep silent and watch the spectators cards, watching them copy my actions and they slap the Jack of Spades on the table!
Feign shock if you are an actor type, or do as I do and project a kind of 'meh' vibe. People will expect you to get out of the bet, but won't be sure how.
'There's terms and conditions - always.' Point the the money and say 'PTO'.