The key card principle is one of the most powerful in magic, and I don't say that simply to exaggerate. I use it on a daily basis, and in a lot of my effects.
The key thing is this: You have to use it in a clever way.
By which I mean..
Don't glimpse the bottom card while it's in your hands, get them to select a card, put the deck on the table, have them put the card on top, and have them cut the deck.
Instead...
Give the deck for them to shuffle, and try and glimpse it while they're shuffling - it's usually possible. If not, glimpse during an all-round squaring of the deck when they hand it back (interrupt them during the shuffle by putting your hand out in front of you, without asking verbally for the deck - this prompts them to stop their shuffling immediately and give the deck back to you, without seeming like you're stopping them at all). Now have them cut the deck. Talk for a while. Remember you have a glimpsed card in the middle of the deck. Have them take a card. Without looking at the deck, lift up half the deck at the glimpsed card. DO NOT look at the deck for any reason. Have them replace the card in the middle, and replace the packet. You now have their card beneath your glimpsed card. Now talk again.
Why does this work? Firstly, you do not glimpse the card while it's in your hands. Big plus. No need to look down. Secondly, you don't cut the deck after the card is replaced. Somehow, this fries everyone. No-one ever thinks of a key card, because they know you have to cut AFTER the card is replaced, and that the card should be on top. But you don't, and it's not. You cut the card into the centre two minutes ago. Which brings me to my third point - time misdirection. Fourthly - they cut the cards, not you, and in doing so, centralise the key card. It's already there, and you haven't touched the cards. And finally - you do not ever look at the deck. You don't glimpse the card you're cutting at. You must not look at it for any reason.
There's only one other thing I should mention - cutting the deck precisely at the key card without looking at the deck or fidgeting. I haven't mentioned the method to this, but to me it should seem obvious to a magician with a little bit of experience. If you don't get it, think about an easy way to do it.
If done correctly, you will never be suspected of a key card.
The key, no pun intended, is this:
Your audience and even magicians have a perception of a key card. If you ask anyone who knows what it is, how to do it, they will say this: "You remember the bottom card, have a card put on top, and then cut the deck". Of all these things, in the above method: The first happens in their hands, and otherwise, you give them no opportunity to even suspect you. So that's ruled out. The card is never put on top. And the deck is not cut afterwards. Furthermore, the deck is only cut following their shuffle, two minutes earlier. So that's ruled out too.
Essentially, what you're doing is this: You take their expectation of what a key card is, and how to use it, and take advantage of it, by using a key card in a different manner, and hence eliminate all suspicion. Take advantage of their knowledge and use that knowledge against them.