If you havent visited your career services office on campus, take time long before graduation to do so.
Introduce yourself to the staff, let them know you will be stopping in frequently (weekly, if not daily in your last year) and utilize their knowledge. They can assist in all your job search (and research) needs.
Before getting your heart set on a major, you may want to discuss with the career services staff the latest information available regarding your choices. If your first choice seems to have a negative 5 to 10 year forecast, its best to research your second and third choices–giving you more options. Consider taking a short career quiz to help narrow down the set of possible career options.