Answer by  
                                                                                          RachelW (932)
                                                            
                          
                          
                                                          Any exercises that target the glutes (the muscles of your butt) will help to give definition.  Lunges may be particularly helpful.  You may just have a flat muscular structure to your butt, which would be difficult to change without implants, which are available but very expensive.  If it is flat because of fat, a healthy diet will help.