Cheating by preparing the deck only works once, and it takes quite a bit of time to accomplish correctly. The cheat simply takes some time previous to the game and puts certain cards in certain places in order to make certain players have certain hands. The typical cheat will organize the deck in such a way that one player will hit a large hand at the beginning and get outdrawn at the very end. This makes it so the other player(s) get very excited about their hand and commit large sums of money to the pot.
The easiest way to prepare the deck is to use a non-symmetrical design on the back, lets say a picture of a baseball player. The cheat can simply organize the cards so that the prepared part of the deck is facing one direction and the rest of the deck is facing the other.
More clever cheats will organize the deck in such a way that they can falsely shuffle the deck and make the desired cards come up on the top of the deck. Other methods can also be used in combination with the prepared deck to make it seem more natural.
You should also be careful of anglers at the table. They may be cutting the deck in a certain way or shuffling the deck a certain way for their partner to use.