As a corset designer and maker I usually recommend that clients go for a corset around four inches smaller than their natural waistline. It sounds like a lot, and it is as temporary as your corset wearing but it can dramatically affect the visual effect.
And here I am going to demonstrate the power of the corset. I have a 28" natural waist and here I am wearing one of my own 24" corset mock up's. The back lacing is not completely closed, but it has reduced my waist temporarily to 25", but see how dramatic the waist has become.