Practice it until you can do it without thinking, then as you walk through the steps, picture what someone might be able to do to mess it up.

For example if they are to pick a card from a deck, they might look at it and try to put it right back immediately.

After you have identified what they MIGHT do, think about what you can do to prevent it. In the example above, you could tell them not just to look at it but to do something with it, (”Now, show it to john”, or “Focus on that card for a few seconds”). Or you could as soon as they draw the card, pull the deck back (smoothly so as to not draw attention to the fact you are doing it).

When you have finished identifying all the things you THINK they could do to mess it up, practice it with those scenarios in mind so they become automatic.

The tricks that fool people the most are the ones you don’t have to think about when you perform them. If you think about it when you are performing then you may be giving away small tells as to when the important parts are happening.