They judge you based off (in order of what I perceive to be most to least important - I've spent a lot of time worrying about applications process so there is at least some truth here) with,
Standardized test scores, your resume (which includes publications, lab work, research, etc.), letter of recommendation, GPA.
They get about a million applications a year and accept like 20 of them so you need to,
- Memorize any sort of formula that might possibly be relevant to a standardized test.
- Work on developing your standardized test taking skills as if you were working on your football skills for the purpose of getting into the NFL.
- Obviously keep your GPA above at least a 3.8 or so.
- Get published, multiple times if possible and better yet as first author.
- Do an REU every summer, and be a lab assistant in the school year.
- Convince someone to write a letter of recommendation for you saying that you'll be the next Einstein.