The core is located in your midsection and appears as a thin sheet that holds your intestines inside; it lies under your 6-pack muscles and under your internal and external obliques.
These abdominal muscles are the closest to your spine and are a tremendous muscle in supporting your body in general.
By far one of the best exercises to work your core is called a plank.
For this, you will get down on the floor using a mat if you choose.
Basically you lie straight down like a plank(hence the name of the exercise)and your hands should be right underneath your face with your arms in a triangular position to where your hands meet directly under your head.
Now one HUGE technique that is unused in the ab industry is pulling your belly button upward and away from your waist band; you should undoubtedly feel this sensation as soon as you complete this.
Then squeeze those lower pelvic muscles high.
Hold this position for 30 seconds while squeezing your abdominal muscles.
If you even want to attempt a more vigorous and strenuous ab variation to this; simply tilt your chin to face your toes and it will work those 6-pack muscles along with all those lower muscles.
Eventually you want your time to go from 30-45 seconds, then reach 45-1:00 as your abdominal strength increases.
By doing these for around 5 weeks, you will see a phenomenal difference in the look and feel of the core muscles and definitely get serious results.
To add more intensity to this and get a unbelievable sensation, try doing other core, oblique, or abdominal exercises back to back and take a minute rest to see best results.