Understand that the key card is what makes the trick and NOT the trick itself.
A key card is one you use to identify either where you are in the deck or how far away you are from a chosen card.
Make sure to memorize its value and suit. Otherwise, you have absolutely no trick!
If need be, mark your key card somehow, perhaps with a slight marker smudge in an upper corner.
Always keep track of your orientation. A key card can remind you that your card is "X" number of cards in front of or behind it. You need to know which one it is-before or after.
When memorizing the key card or counting the number of cards away from it, always use patter to distract the spectator.
Avoid using the same key card several times in the same routine. People WILL catch on.