I use four different exercises to develop my abdominal muscles. The first two will be know to most people who workout at a gym. The latter two exercises while being less common are probably more effective in developing your six pack.
Firstly I like to do sit ups on a decline bench. Rather than doing huge sets at a fast speed, I use a much slower exercise cadence of 4 up, pause for 2, then 4 down, hold for 2, then repeat, for 10 reps. all all stages of the exercise, my abs are being worked. By the end of the set my mid section is burning. This exercise works the upper abs in particular.
After a quick stretch I then go on to leg raises. My gym costs only $1 per visit so doesn't have all the equipment of more expensive places. For my leg raises I hang from a chin up bar and simply raise my knees towards my stomach. Again, using a slower cadence ensures a powerful burning sensation in the abs. Leg raises target the lower abs.
I usually only do two separate abs exercises on any one day. Next I will talk about two possibly less well known abs exercises.
The abs wheel, also known as an abs roller is a piece of exercise equipment that you can use to give your abs a serious workout. What I love about the abs rollers is that you don't have to do massive sets of 30-40 reps to get a good burn in your abs. Literally after 10 reps or so you know you've hit your muscles properly.
This is an excellent You Tube video showing you the correct form for exercising with an abs roller.
Finally I use the abs vacuum exercise. It is a really interesting exercise and versatile too. Once you get the technique down you can do it almost anywhere, e.g. sitting on a chair watching TV, standing in line or lying down at the gym.
The basic technique is to breath but rather than raise your chest, you raise your belly. Then exhale lowering your belly. Then rather than inhale again, you suck your belly button towards your spine which creates a kind of vacuum effect in your lungs. The more you suck your tummy in the tighter the vacuum becomes. Then when you relax, the vacuum is released and air floods back into your lungs again. It took me a while to feel this exercise but you know when you start to get it, it just feels right. This You Tube video provides a demonstration of the abs vacuum.