"Here is the best-kept secret of the trade: dental floss. It is just about impossible to cut a cheesecake cleanly with a knife; too much of the precious filling sticks to the sides of the blade. You can lick this problem by taking a long strand of dental floss, stretching it taut, and gently pressing through the cake. Don't try to pull the floss back up through the cut you have just made; simply pull it out when you reach the bottom. Either waxed or unwaxed floss may be used, but avoid mint-flavored."
From "The Joy of Cheesecake"