There are a couple of movements that are very basic and yet give many people the results they are looking for. The first movement is the basic crunch. These exercises are better for the body then sit ups because they do not strain the back as much. A crunch is performed simply by lying on your back and raising the upper portion of your body a few inches off the ground, just enough to feel the strain in the stomach muscles. The key to these movements is to be sure your stomach is doing the work and not your head, or arms. Also, it is important to keep everything straight, do not lift the head more then the shoulders or back, for more information and detail on performing this movement properly there are many resources available on the internet, in books and also in videos. Another movement which is also great and should be done in conjunction with the basic crunch is a side crunch which works the oblique, or as some people call them "love handles" This exercises are performed the same way as the crunch, only the body is twisted slightly to one side. Again to get a more accurate description of how to properly perform this movement there are many resources available.
Along with working out the mid section of the stomach and sides, a person should include several different exercises to get a flat stomach by way of exercising the whole stomach, which includes the lower section and upper section of their stomach as well. It will do a person no good to only focus on one specific muscle group of their stomach. There are many places to find exercises to get a flat stomach including the internet, library, book store and in videos. With so many different options a person may have trouble deciding which exercises would benefit them the most while trying to get a flat stomach. The best suggestion in this case is to try several different ones and decide what works for the individual. One exercise may work great for one person, but be too strenuous for another. Or, one person may be in better shape and be able to do several different exercises that a beginner can not do, at least in the beginning and may just need to concentrate on one movement, instead of a series of different movements in one workout session.