My 8yo son has qualified for the Study of Exceptional Talent by scoring at least 700 on the math section of the SAT. His verbal and writing scores were in the 400s, which are also good at his age.

He prepared as follows. After finishing Honors Beginning Algebra at EPGY I challenged him to get a 600 on the math SAT and mentioned 700 as a "reach". He read an SAT math book by Richard Corn as well as The Official SAT Study Guide and a Kaplan book, and he took three practice tests (just the math sections). He has participated in MOEMS (Math Olympiads for Elementary and Middle Schools) for 2 years.

Besides his scores, I am pleased by the effort he put forth to prepare and to sit for a 4-hour exam. My advocacy of a grade skip in math has failed so far (skipping 3 grades to 8th grade Algebra I would be conservative), but so far our after-schooling appears to be successful.

I used to do a deal with my 12 year-old when he was that age: I would do his math homework (faked his writing) and in exchange, he would do his algebra.