Here is a simple idea to install a memory effect in the future.
Effect
In a borrowed, shuffled deck, a spectator chooses a card (for example, the ten of hearts). You reveal your prediction: "You will choose the ten of hearts. I will always remember it and you too."
You add that you really mean that : you will never forget her card and, if you meet again later (whether it's 5 minutes later or 50 years later), she can ask you to name her card again. You will always be able to do so.
Method
Here, the real effect is the "eternal memory" of the chosen card; the prediction effect is only a pretense to designate a card that you will remember forever. Of course you could simply force the same card to everyone, but if you do table hopping or if you work for a group that might talk among themselves afterward, always forcing the same card is not a very viable approach.
My idea consists in using the first name of the spectator to tell you which card to force. Therefore, when you met her again later (whether it's 5 minutes later or 50 years later), she will introduce herself again, unknowingly reminding you of her card. The system is base on a stack that you already know. If you don't already know a stack, this other article should convince you to embrace that tool: Why a stack?
When the person gives you her first name, concentrate on its initial. Take the position of that letter in the alphabet (because of course you know the numerical position of every letter in the alphabet, right ?! Right?!). This value will be between 1 and 26. Now take the card at this position in your stack, that's the card you're going to force.
Let's see some examples, with the Mnemonica stack as a base. If the spectator's first name is Melania, the initial is M; M is 13th in the alphabet; the 13th card in Mnemonica is the queen of clubs. If the spectator's name is Donald, the initial is D; D is 4th in the alphabet; the 4th card in Mnemonica is the three of clubs.
Once you know the card associated to her name, take a paper and write the corresponding prediction: "You will choose the [card's name]. I will always remember it and you too." The phrasing is ambiguous: it could mean that you will never forget her card and that the spectator will never forget her card and/or this moment; it can also mean that you will never forget her card and her.
Continue by forcing the card to the spectator. Any convincing force will do. Personally, I use Dani DaOrtiz's variation of the classic force described in his Utopia DVD. An easy solution would be Balducci's Cut Deeper Force.
The spectator takes her card, you reveal the prediction and the main effect is over. You tgenadd that if you meet again later (whether it's 5 minutes later or 50 years later), she can ask you to name her card again. Of course, if you meet again later, she will introduce herself again by giving you her first name (or you will ask her) and she will unknowingly give you the key leading to her card.
Notes
You don't gave to use this first name idea for a prediction effect. You could meet someone, ask her name, make her "choose" a card (in fact forcing the right card) then use this card for any effect (ambitious, card to pocket or to wallet, etc.). After the main effect is done, just tell the person that you will never forget her card and that she can quiz you next time she meets you. Another approach is not to force the card: the person introduce herself, you deduce the corresponding card and if, when you perform a card trick to her, chance has it that she picks the right card, take advantage of the situation to tell her that you will never forget.
You can of course borrow a deck for this effect. All you need to do is to locate the corresponding card then force it. If you're using your own deck and it's stacked, it will be even easier to locate the card.
Asking the person's first name when you meet her again can seem paradoxal, since you pretend to have a good memory, but it can easily be justified. Simply say that, in your head, you have a big list of people associated with cards, meaning you didn't necessarily memorise the first name linked to each face.
You can push the idea even further by integrating the family name in the code. If the initial of her family name is in the first half of the alphabet (A to M), use the system as described before. If the initial of her family name is in the second half of the alphabet (N to Z), add 20 and use that number as a stack number for the card. For example, if the spectator's name is Melania K., you translate M into 13 like described before; K is in the first half of the alphabet so you don't add 20; for Melania K. the card will be queen of clubs. But if this spectator is married to Donald and her name is Melania T., you still translate M into 13, then you add 20 (T is in the second half of the alphabet) and obtain 43; Melania T. would get the ace of clubs. This way, Melania K. and Melania T. have different cards so the method is even more hidden even if they chat together afterward.
Instead of a playing card, you can use a symbolic deck or an ESP deck containing 52 different cards that you can easily memorise like a stack. In fact, you could use any ensemble; you just need to memorise it, it needs to contain more than 26 elements, and you must be able to force one of the elements to the spectator.