Be one the lookout out for any new sit up machines that hit the markets. Also wear a thermal belt when doing ab workouts they will give you greater cut. But yes lower abs are are the hardest parts to hit.

Situps — anyone with half a brain would learn how to do them properly

Interesting tidbit, but the external obliques actally fire more during a pushup than they do a crunch. Depending on what part of the abdominals you wish to work, there are several different strategies you can employ. The internal obliques and rectus abdominus fire during crunches, and they are a valid exercise for that purpose, but the external obliques are better served during stabilization of the body during limb movement.

Intensifying abdominal exercise is largely related to lengthening the lever arm. As an example of a progression for this, I will use your hated crunches because it is a straightforward example. If crunches with the arms crossed become too easy, the arms can be moved to the head, and then above the head, then adding weight, etc. These same principles can be applied to exercises like bridging and planks (both of which are excellent isometric abdominal exercises)

The nice thing about planks and bridges is that you can set the intensity through duration and creation of an element of instability in the exercise, either through a ball, a wobble board, or if you are really lucky, a vibration platform. Movement of the limbs during isometrics also challenges the abdominals to maintain stability, which is their most important function.

Try laying on your back and doing knee crosses. Cross your elbow to your opposite knee 10 times, then switch sides. Really works.!!!! 3-6 times will work too……I am overweight but my tummy is flat from doing this exercise.