Side Plank!
While you cannot spot reduce (lame, we know), you can tone up your abdominal and oblique muscles, which basically creates an au naturel corset of sorts. But forget crunches - the ultimate waist-toning workout is a plank. Take it to the side and you're targeting those obliques. Try different variations, like side plank ups and side planks with an oblique twist.
[Photo via @miniyingyoga]