a) Adequate preparation in mathematics. Applicants should have multivariate calculus. Linear algebra, real analysis and probability theory and/or statistics also looks good.
b) A good grade record. This is not precise as standards vary widely among schools.
c) Informative letters of recommendation. It is helpful if these show the applicant is creative and enterprising.
d) Of the GRE scores we focus mainly on the quantitative score in percentage terms. Anything over 90% is fine; below 85% raises questions.