Focus on your natural waist and the fullest part of your hips. Do not size down, as this can cause discomfort and "muffin-topping" at the edges.
Stores like Bare Necessities or Vermont Country Store carry traditional and firm-support styles that are harder to find in mainstream malls.
Ideal for smoothing the thighs and preventing chafing, these are often preferred under trousers or long dresses.