Eating anything 20-30 minutes beforehand will do that. Even eating a meal more slowly will result in eating less due to feeling sated sooner. The first bullet point here is relevant: http://zenhabits.net/5-powerful-reasons-to-eat-slower/
If you're relying on iceberg lettuce in your salad you're really doing it wrong.
Have a look for some mediterranean salad recipes, try putting some cherry tomatoes in for sweetness, a light drizzle of olive oil and swap the lettuce out for rocket and see how it goes.
Lettuce, like most leafy greens used in salads, is mostly roughage. Iceberg lettuce isn't as tasty but otherwise is fine. And iceberg lettuce is cheaper.
Lettuce, tomato, avocado slices, chopped green onions, and an anchovy or smoked oysters with vinaigrette is fantastic, regardless of what kind of green is used as the base.
However, most American salads are loaded with decidedly non-healthy items, so a 200-300 drop in meal calories would be offset by the salad itself.
Better just eat them to increase vegetables, then. (No iceberg!)