In 'how to cheat your friends at Poker: the wisdom of Dickie Richard' by Penn Jillette, Dickie (a 'professional' poker cheat) explains that he created his own system for marking cards by starting off by only marking the ace of spades with his thumb nail. On every deck of cards that fell into his hands after that, he always made the same mark on that card so he always knew where the ace of spades was.
Then he devised a different mark for a different card, say thumb nail nick in a different part of the card to denote the ten of clubs and again applied that mark to every deck he played with.
Over time he created marks for every card in the deck but because he developed the system over time and there was no logical order to it, it was virtually impossible for anyone to work out. The only key to the system was his memory of what each mark meant.
I'm not saying that you should create your own marking system from scratch if you're only going to use a marked deck occasionally, but if it was something you were going to use a lot, or if you wanted to become a full-time card cheat, it's a pretty clever idea as it means you can mark cards (maybe not the whole pack at once) even if you're borrowing them from someone else.
I can't remember exactly where in the book he talks about his system but it's worth reading the whole book just for the fun of hearing about the crazy and dangerous stuff the guy got up to and it's got loads of useful day to day tips such as what to do when people start beating you up for cheating at cards (lie down and start crying like a baby- most people don't have the heart to keep on kicking someone in that state) and the best way to secretly have sex with the wife of the guy who's organised the poker game!
