If you are having trouble with moving your waist properly, move your legs further apart and plant them, bend them, and focus on one foot, moving one leg in little circles starting with your hip. This will help moving the waist at the right rhythm.
Larger hula hoops will spin slower and be easier to use.Heavier hula hoops will also be easier to use, but they shouldn't be so heavy that you have trouble keeping them up.
Hula hooping is a great way to spice up a traditional abdominal workout. If you're tired of doing sit-ups, try hula hooping instead.