I must congratulate you on being one of the lucky few with a slim, trim waistline. You can keep it that way by doing certain toning and firming exercises two to three times a week. They will also aid you in maintaining a flat stomach. Try the following:
Pilates Criss Cross-this exercise stretches and strengthens the abdominal muscles and helps prevent those unsightly "love handles."
Pilates Side Plank-also an oblique exercise, it strengthens the core abdominal muscles, hips, thighs, waist, arms, and shoulders.
Pilates Bridge With Leg Lift-an excellent way to strengthen the pelvis, obliques, hamstrings, lower back, and gluteals.
While these are not the only exercises you can do, they are an excellent first choice.