For any muscle in your body you always have to change exercises otherwise it becomes accostumed to that particular exercise. Muscle memory.There are a bunch of ab workouts that you can do besides the typical crunches. For my personal workouts I use the p90x ab workout. It involves 11 different ab workouts and none of them are the typical straight sit-up. There have v- roll ups, mason twist, scissors, bicycles, cross legged situps, side v-crunch, heels to heaven etc... each of them 25 times.

the plank!....lay on the floor and get into a possition ware ur elbows and ur tows are holding u up!....keep ur back straight...works on the muscles endourance...rips u like none other....but also lay on ur back...lift ur legs lift ur legs lift ur legs....it will hurt after a while

Roman Chairs: hard to explain but you can google it. They are great for isolation, including your obliques and there is no way to cheat:)

Core work with a medicine ball will also do alot for the 6pack

try the russian twist the only place you will feel it is in your abdominals it also hepls to eat low fat food

Use a medicine ball with situps, crunches. and a good one is sit down lift your feet in the air take the ball and twist your abs. This helps a lot. And burpies. Those are go down in pushup position have your feet jump to your hands and jump up and repeat.

you can hold your feet and arms off the ground for a long time on your back. it really works your abs

the roman chair is excellent. however, you generally have to go to a gym to use one. do-it-yourself ab work will require you to go to a sports store and ask if they have an ab wheel. if they do, have someone show you the right way (and do three sets of 20 three times a week, you'll be only able to do about 5 at first, work your way up). my other suggestion is getting "gravity boots" and doing sit ups upside down.

Lay on your back and lift your legs high enough so your bottom doesn't touch the floor. Do the same with your arms so your shoulders don't touch the ground.