Those exercises are good, but i find they tend to work the hip flexors rather than the abs, and this can decrease your flexibility severely
Try lying on your back in a tuck position, with your head off the floor and tour knees up to the ceiling, keep your back still, rounded and on the floor, and slowly straighten your legs as close to the ground as you can, when you are fully extended make sure that you squeeze your bottom, do these slowly in about reps of 20-30 depending on your strength. You can also do this one leg at a time, circling forward or backwards, or do both legs like a bicycle motion.
A really difficult stomach exercise which i like is dish circles :- lie on your back with your shoulders rounded off the floor and your legs straight out infront of you raised about 6 inches off the floor, squeeze your bottom and hold it for ten, then slowly do a giant circle to the right with your legs, do ten circles and make them smaller every time, so that by the last one your cirlce is tiny, make sure your shoulders neck and head are rounded and off the floor at all times, and there is no hollow in your back. Also make sure you repeat this to the left to work the opposite muscles. This exercise is great because it targets the whole of your abdominals, the bottom, middle, top and sides, which alot of people tend to forget about.
Hope this Helps!!