Here are four exercises you can do, one minute each, 3 or 4 times a week for stronger more firm abdominals.
Abdominal exercise 1 - crunches one minute
Lie down on a soft mat or carpet. Bend your knees so that your feet are flat on the ground. Clasp both hands behind your head to support your neck. Slowly bring your head straight up, not forward, pulling from the abdominal area as you exhale. Your head only needs to come up far enough to feel a complete contraction of the abs. Then allow your head to slowly return down to the floor and repeat. Be sure to do each repetition slowly and controlled, expelling a breath on each contraction. Do as many crunches as you can in 60 seconds.
Abdominal exercise 2 - reverse crunches one minute
Start in the same position as with the crunches, but this time cross your legs at the ankles and slowly bring the knees up into your chest and straighten out your legs, but still with a slight bend as your arms lie flat on the floor, palms down beside your body. Lift your pelvis off the floor an inch or two using only your lower abdominal. Make sure you feel the lower abs working and not the momentum of your legs swinging back and forth. Do as many as you can in 60 seconds.
Abdominal exercise 3 - oblique crunches one minute
Start by lying on your back on a mat or soft carpet with your feet flat on the floor. Take one leg and cross it over the other leg so the crossed leg is resting on the thigh of the other leg. Bring the arm of the bent leg behind your head while the other arm rests on your stomach or on the floor, which is an easier version. Slowly bring the arm that is behind your head up as you twist so the outside of that elbow goes towards the outside of the opposite knee. Slowly lower down and repeat. Do as many repetitions as you can in 30 seconds. Then switch sides and repeat.
Abdominal exercise 4 - slow bicycle one minute
Lying on your back again, clasp your hands behind your head. Bring both knees into your chest and then extend one leg fully as you push out with the heel, simultaneously as you slowly crunch and twist bringing your opposite elbow to the opposite knee. After your opposite knee and elbow touch, or come close to touching, slowly continue to do the same with the opposite leg and arm. Picture yourself riding a bicycle very slowly while lying on the floor. The key to this exercise is to do it very slowly to a three count allowing the upper abs and obliques to work in unison. Repeat for 60 seconds.
Practice this ab program three to four times a week and your abdominals will be rock hard before you know it.