The best way to strengthen your back muscles is to work on strengthening your core muscles (abdominal area). Pilates, mild crunches, exercise balls, etc. work great for this.
Our skeletal systems are held together by a very complex and interconnected system of muscles, ligaments and tendons, and it's normal to have some curving when you sit or stand in various positions. It typically only becomes a problem when it's severe or assymetrical/twisted such as scoliosis, and what you are experiencing is likely normal.
