"The consumption of a food typically leads to a decrease in its subsequent intake through habituation — a decrease in one’s responsiveness to the food and motivation to obtain it. We demonstrated that habituation to a food item can occur even when its consumption is merely imagined. Five experiments showed that people who repeatedly imagined eating a food (such as cheese) many times subsequently consumed less of the imagined food than did people who repeatedly imagined eating that food fewer times, imagined eating a different food (such as candy), or did not imagine eating a food. They did so because they desired to eat it less, not because they considered it less palatable. These results suggest that mental representation alone can engender habituation to a stimulus."
Update 1 (17/7/2011)
Paul Rozin, Sydney Scott, Megan Dingley, Joanna K. Urbanek, Hong Jiang, and Mark Kaltenbach argue that subtle changes in the way different foods are made accessible in a pay-by-weight-of-food salad bar in a cafeteria serving adults for the lunch period can play an important role in reducing obesity. They found that "making a food slightly more difficult to reach (by varying its proximity by about 10 inches) or changing the serving utensil (spoon or tongs) modestly but reliably reduces intake, in the range of 8–16%". They conclude that "making calorie-dense foods less accessible and low-calorie foods more accessible over an extended period of time would result in significant weight loss".