I learned this one from my sister (who was a kindergarten teacher). If a clothing stain has an oiliness to it (examples: tomatoes, oil based salad dressing), gently rub some Murphy's oil over the stain, then launder as usual. Do NOT put this article in the dryer...let it air dry. That way, if the stain needs another round of Murphy's oil (it rarely does by the way!), the dryers' heat won't have heat-set the stain.
This product has saved many an article of clothing at my house over the years!