Clue is a classic 50-year-old detective game. You can get the upper hand by quickly eliminating some of the 324 possible combinations of "whodunit with what and where."
Mark off, on your detective notepad, the cards that you hold in your hand.
Put a person's initials next to the card he shows you when you ask for a clue. For example, suppose that player A shows you the knife. Mark his initials next to that card.
Write down the initials of the person who shows another person a card when you're sure which card it is. For example, if a player asks someone for the knife and Professor Plum in the kitchen, player C shows a clue and you already know that A has the knife and B has the professor, then you know that player C has the kitchen.
Remember (or mark off) the cards that you show to each player. If a player forgets and asks you for the same card twice along with another card you hold, show her the card you previously showed her.
Keep a poker face. Be mysterious about which cards you hold and don't hold.