For such a complex issue, unwanted fat is surprisingly simple. Fat builds up because more energy is taken in than is used up. The first and best solution is diet and exercise. If you use more energy than you take in, fat deposits will reduce. However, modern life is not so simple and there are a range of available options to help the good work of diet and exercise along a bit.
There are mechanical and surgical methods to help with this process, ranging from the use of radio waves to break up fat cells painlessly, to liposuction. What is right for you will depend on your personal situation.