Some good exercises for trimming and toning the waistline are side plank-ups, round backs and fly-ups. All of these exercises can be done at home with no specialized equipment.
Side plank-ups focus on the front and side abdominal muscles as well as the oblique muscles and inner thighs. Round-ups are also a solid workout for both the side and front abdominal muscles, but may put undue stress on the lower back. Fly-ups work strictly the front abdominal muscles, so they are a good choice for those wanting to isolate this group of muscles. All of these exercises offer a reasonable amount of resistance using nothing other than body weight and gravity.