All right, the part that I bet most students who read this are zooming straight to. How do you get into Caltech? What can I do to my application that will make my chances of admission higher?
The number one question at the Caltech Info session I attended revolved around test scores. "What kind of SAT score do I have to aim for?" Your SAT score and other standardized test scores probably mean squat, and are not indicators of how well you will do at Tech. I knew folks who got 1600 on their SAT's (this was before there were 3 parts to the test) and they flailed through all 4 years of Caltech. The Admissions committee knows this. So don't bother retaking the SAT a billion times to get that perfect score. If you scored relatively high, then that's all that matters... you just need to prove you are smart enough to take a test, not that you have mastered standardized testing.
The most important thing that you can show on your application is that you have challenged yourself as much as you possibly can with the resources around you. This means that you should have taken the most challenging classes at your school. If your school offers AP's in sciences, you should take them. Having gone to the community college nearby for classes that your high school doesn't offer is also a big plus.
Me, I went to a magnet program in high school, and I have a feeling that was a big factor in my admission. I sought out the hardest stuff I could do in my school district.... I took at least 7 science/math AP tests, and did pretty well. I ended up being an average student at Caltech. On the other hand, my husband took 3 science/math AP classes in high school because harder classes weren't offered. His high school science classes were a fraction of the difficulty of my classes. But he was admitted to Caltech early, had a much easier time grasping the concepts than I did, graduated with honors, and is now a graduate student at Harvard. I think that the admissions committee could see from the rest of his application that he was a hard worker.
This brings me to extracurriculars. What I said above about challenging yourself holds here as well. This is where I think my husband made his application. He was an Eagle Scout, and the section leader for the saxophones in his extremely successful high school marching band. Both those things took a lot of time and dedication and hard work.
Almost everyone I knew at Tech had something interesting to put in their extracurriculars. Some of us did research over the summers. (I think that's what got me in; I worked at Fort Monmouth for 3 summers doing engineering research.) Some of us goofed around and built wacky things with our friends (a friend of mine built a remote controlled submarine). None of us were simply classes classes classes. We challenged ourselves outside of school as well, and were able to juggle everything.
So those are the important things in your application. Which means that by the time you are a senior and are applying, there's very little you can actively do to pad your application. It all depends on the stuff you've already done.. have you taken the hard classes? Have you challenged yourself extracurricularly? It's a little late to fix that now! Great SAT scores won't make up for slacking off in school and doing nothing over summers but video games.