Some advice on your graduate school applications from someone who's done it (thankfully only once) and advised other people to do it (a few times ):
1) Get your applications in early. Most schools won't admit it, but admissions are very often a rolling process. I know students who were admitted to schools 2 months before the deadline.
2) Get in touch with the professors you think you'd like to work for. You should ideally be doing this a few months ahead of applying, but it's never too late. Don't expect a professor to respond to the email you sent the day all of the applications to their program were due.
3) Show your creativity, independence, research experience, and enthusiasm. Grades are subjective, test scores are mostly worthless. The most successful graduate students I interacted with at Caltech were not the ones with straight A's in undergrad - they were the ones who had been developing their experimental skills, taking challenging courses (where they might get a bad grade, but learn a lot), and actively thinking about bringing new ideas into their projects.