"... as a losing opponent essentially has no path to victory, even with lucky rolls. Your goal is to play conservatively, lock up more resources, and let the other players lose by attrition. If you want to see these people again, I recommend not gloating, but simply state that you're playing to win, and that it wasn't your idea to play Monopoly in the first place."
From "How to Win at Monopoly and Lose All Your Friends," recommended by Throwing Things. The trick is to create a housing shortage.