don't just join a bunch of clubs, you have to show dedication. college counselors know that students tend to do that just so that it'll look good on their apps. yes, they do want well-rounded individuals, but they also want students who are going to make a difference on their campus and contribute skills, passion, etc.
find some things that you enjoy or are interested in and dedicate yourself to it. for example, if you're interested in ballroom dancing, start a ballroom dancing club, hold events, be active. do something that hasn't been done before. put yourself in leadership roles. don't just be a member of a bajillion clubs.
get to know your teachers (if you haven't already). they're the ones who are going to write your recommendations, keep that in mind. i've read some pretty sh*tty recommendations. they don't really help.
take challenging classes.
the obvious - keep up a good gpa. further, colleges don't just look at your cumulative gpa, they also look at grade trends. having crappy grades in your frosh and soph years sucks but if your transcript shows you worked hard to get better grades as you got older, it's impressive. get good sat scores. take a lot of practice sats so you can become more familiar with the format. read a lot to increase your vocab and help your reading comprehension. it should help your writing too.
sorry, i wrote alot

good luck, pm me if you have other questions...
cornell, brown and amherst are probably gonna be the toughest to get into.