Make careful plans for entering college, if you haven't already. Apply to more than one college and get accepted. Apply for financial aid and visit more colleges, if you need to. Take the SAT or ACT if you haven't already.
Stay focused on grades and academics. Often, acceptance into college is contingent upon your continued solid performance in academics and extracurricular activities. If your grades slip substantially, it will show up on your transcript, which you are often required to send to your college at the end of senior year.
Maintain a heavy, challenging course load. Take a couple advanced placement courses or a few courses at a local college, if they're available to you. Stay active and involved in extracurricular activities and volunteer work. Try out an internship or activities related to your career path.
Avoid procrastination and strike the proper balance. Putting off school duties in order to have a bit of fun is normal, but make sure to set aside time to get things done. Still, getting yourself too overwhelmed with school work can leave you emotionally and physically run-down.
Enjoy spending time with your friends and family. This should always be the case, of course, but savor these relationships in particular during senior year, since after this, you will not see them nearly as often if you're leaving the area for college or work.