Some perform a vacuum in conjunction with bending over, reaching overhead, or when lifting heavy weights, although the benefit and healthiness of this is disputed (some advocated just tensing the midsection, with primary focus more so on the lower back).
Increasing the range of motion (how far it is drawn in), the intensity (very little can be done, other than resisting gravity) or the duration are the methods of making the muscles stronger.
Performing flutters or Tabata intervals of high intensity and rest similar to Kegel exercises is also common.
[1] A method of developing long-term daily endurance in the muscles is to tie a string around the waist at the bellybutton level.
The significance of this is thought to be that hypopressive exercises increase the base tone of the abdominal wall (as well as the pelvic floor) and hence reduce resting waist circumference.