Study hard. This is the foundation of a good high school record and will also endear you to your teachers.
Participate in extracurricular activities. Colleges look for well-rounded students who do more than just get good grades.
Do community service. This will make your application even more well-rounded.
Take difficult classes, especially Advanced Placement (AP) classes. Colleges want to see that you won't shy away from academic challenges.
Study for standardized tests long before you take them. The more you're prepared for the actual test, the better you'll do.
Cultivate good relationships with your teachers, who will be writing recommendation letters for you.
Spend a lot of time thinking about and writing the essays on your college applications, as these are weighed heavily by admissions committees.
Be as natural as possible during any admissions interviews you might have to undergo: colleges are looking for human beings, not robots.