Everybody wants a six pack, but most people train their abs totally wrong. Often people do endless reps of extremely easier exercises and wonder why they don't get results. Well, if you want nice abs you have to be lean and your abs have to be strong. Getting lean is simple, stop eating like a pig and start exercising to burn fat off your ass. Kettlebells, sprints, circuits or complexes are great ways to burn fat.
As for improving the abs themselves, just like any nice muscles they have to be strong. To strengthen your abs you need tension. Tension in the abs can be created by doing a compound movement like squats or deadlifts with a heavy weight. When doing such movements you have to tense your stomach great in order to prevent from collapsing under the weight. The other way to create massive tension in the abs, and thereby strengthen them, is to work on very difficult abdominal movements.
One of my favourite abdomial exercises are ab wheel, also known as evil wheel, roll outs. These create great tension in the core and are very safe. Below is a video of an extreme example of ab/evil wheel roll outs. Check this out to give you an idea what strong abs are: