Creases can be caused by excess fat. When you sit down see if the creases are where your fat rolls are.
I used to have a minor belly roll, just at the bottom of my belly when I would sit down, the best way to get rid of it is honestly....crunches. all kinds of crunches help different parts of your belly. I usually start of with a set of 50 normal crunches, then 25 leg lifts, then a minute plank and then 30 second planks on either side. Total time = 3 mins. Try this, you'll feel the fat burn away as you do it