A key component of any workout, core training & core stability exercise is going to not only help you develop a lean, tight mid-section/trunk, it's also going to greatly play a role in preventing injuries in the future.
There are many different ways you can go about doing core training, from performing simple sit-up exercises and variations to using plyometric type of activities that will also utilize many other muscle fibres in the body. Core stability exercise programs are worth their weight in gold!
Generally it is best if you can figure out a way to work the highest number of muscles in the body because this is what will help you see the fastest results in the shortest period of time. As this area does comprise many muscles, you can easily accomplish this.
You must keep in mind as well that because the muscles in the mid-section of your body are worked any time you perform a regular weight lifting exercise (squats for example) that you must also ensure you are getting enough rest in between sessions as well.
This is crucial if you want to see increases in strength development. If you are never allowing your body to recover between sessions, you are only going to continue to break down the muscle fibers, causing them to become weaker and weaker instead of getting stronger.
Finally, remember that it is always important to work smarter rather than harder. With this type of training in particular, many individuals get into the mindset where they think quantity will be the best way to go. They'll start performing hundreds of reps of an exercise only to find that they aren't really progressing. So rather, really focus in on the muscle and make sure you are not cheating yourself of working it to its fullest.