Check out the classes offered at your high school and make sure that you plan your yearly schedules so that you have the prereqs needed to take a dream class. My daughter found a class in high school that changed her life. I know that with budget cutbacks schools have less resources to offer classes other than the basics, but try to find a class that is different from the others. When my daughter registered for her sophomore classes at the end of her freshman year, she registered for a music theory class since she was ahead in math and science (and was very good at music). Over that summer though, she volunteered at a rehabilitation hospital and got interested in medicine. She found out that her high school offered a Human Biology class and in the fall she managed to drop the music theory class and somehow got into the Human Biology class--the only Sophomore out of the Juniors and Seniors in several sections of the class! It was the best thing that she could have done...it was stimulating and difficult--just what she needed.
Because of that class she was eligible to take an EMT (emergency medical technician) class as a senior. She was SO afraid to take the EMT class because the thought of having to ride-along on ambulance runs scared her, but she decided to take the chance. Overcoming the fear of riding along on the ambulance allowed her to put her anxiety into context so she could meet other fears head-on...going away to college, meeting her college roommate, flying home alone on breaks etc... She passed the required exams and now volunteers with the EMT's on campus. Encourage your child to step outside of their comfort zone...They will have to do so many times as an adult...now is the time to practice (while you are around to support them!)
Encourage them to take difficult classes that interest them during the senior year. Senioritis hits everyone, but if they are busy with stimulating classes it doesn't hit quite so hard so early in the year. Plus, it is a foreshadowing of college...many kids never studied in h.s. and it is difficult to get into the swing of studying if they didn't have to work hard in h.s.
If your child is not sure what fascinates them, get them involved in some kind of volunteering (that THEY are interested in) during the summers. Something might just click for them...it might not click the first time but it may lead to another revelation. (My daughter realized that summer that being a physical or occupational therapist was not for her--but she is a sophomore studying biomedical engineering now and loves it...still helping the handicapped population, but on a totally different level.)
If possible, encourage your child to take some classes with older students. Both of my kids had great friendships with older high school students. It hurt when the older kids graduated, but my kids realized they could look forward to college too. They got the inside scoop on colleges, admissions, and senioritis so they knew what to expect. Mostly they learned not to procrastinate! When the college kids came home with stories of their college, it gave my kids a heads-up about what college would be like and motivated them to stay focused on school.