I really don't know much about this. I myself teach at agood school, but not a top notch one, and we are always starved for talent. But I guess the criteria are the same everywhere, so I guess I know what to do to prepare, I just don't know how competitive the top schools are at the grad level, so I don't know how likely one is to get in by doing them.
It is not so much a matter of doing special activities or projects, or endearing yourself to professors, but just a matter of working hard to learn as much as possible, and trying to polish ones ability to do research. Although we talk otherwise in education, in recruiting we tend to behave as if math is an inherent talent rather than an acquired skill.
so we are always looking for that person who sees deeply into the subject they encounter, and who comes up with insightful comments and questions, and who finds creative approaches to problems, and who then pursues them successfully, with strong computational skills.
I guess the only part of this that you can acquire is the knowledge by sustained study with the best masters and books, and strengthened computational abilities through persistent practice at working out examples.
In my own career, I made a quantum leap by spending 2 postdoctoral years at harvard, trying to soak up as much as possible from people like david mumford and phillip griffiths, and heisuke hironaka.
i went to their lectures, asked them questions, and read the papers they referred me to (I did not read as many of those as I should have in some cases.) i volunteered to give talks and invited them to attend.
eventually i got better, and they seemed to notice it, and they helped me. but as to getting into a top grad school, i think the goal should rather be to get into the grad school that is at the right level for you, that offers courses in what you want to study and that has professors you can learn from.
for me harvard would have been a disastrous grad school, but it was an ideal postdoctoral experience. utah was perfect for me as a grad school, because it had herb clemens, the advisor who helped me find and improve my research abilities, and gave me an appropriate problem, and helped me learn to solve it.
before that my stay at brandeis helped too, by contact with brilliant and accessible professors like alan mayer, paul monsky, robert seeley, maurice auslander, david buchsbaum. i learned much more at brandeis than as an undergrad at harvard, because the professors at brandeis seemed to notice us and try to teach us. we were run over roughshod at harvard undergrad, by professors who ignored us or made us feel we were wasting their time, a really awful experience.