(a) Straight As
(b) (if applicable) Take all AP courses available at your high school
(c) (if applicable) Take additional college courses through community college dual enrollment or online school
(d) (if applicable) Join the math club, honors society, run for some kind of student council position, or other academics-oriented or leadership-oriented club role that doesn't require a hefty buy-in
b-d may or may not apply, depending on your school, school district, and state.
I don't think the difference between dedicated tutors and individual studying with Khan Academy is a large difference. The majority of low scorers score low because they're either lower on the intelligence scale or didn't spend time individually studying.
60% of 750 - 800 math SAT scorers are Asian
https://i0.wp.com/www.brookings.edu/wp-content/uploads/2017/...
I looked through the list of USAMO finalists in recent years and > 80% have Asian surnames.