Although it is almost self-explanatory to say, being grade conscious is always very important. Universities are first and foremost educational institutions and they care about your abilities in the classroom. On that note, the classes that you do well in should be ones that you enjoy and that challenge you. I recognize that this is difficult to do in a high school setting, but take advantage of any freedom you are given in choosing your classes. If you like a certain subject then pursue that by taking an extra course. Do what you like to do, and at the same time challenge yourself. AP or IB classes are obviously a good idea and highly recommended.
Additionally, if you are academically interested in something- take an initiative and delve further into that topic! This means independent studies, submitting papers, etc. I wrote a research paper for my AP World History course sophomore year and decided, on a bit of a whim, to submit it to the Concord Review, a quarterly which publishes history papers of students from around the world. To my surprise I was published the fall of my senior year, and I believe this was one of the strongest parts of my application. Colleges like to see this level of interest and more importantly, independent motivation. Academics in college are all about working hard and being on your own so show your universities that you are a step ahead of the average student and are working on a college level in high school. Moral of the story: take chances with your academic interests and do not forget to push and challenge yourself. Try to take it one step further.