Volunteer Work! (MAGIC)!!!!
Depends on where you are applying, but in general
SAT ... do well. One of the secrets to doing well is getting good rest the night before and DON'T PANIC. Brush up on things, but don't "cram" the night before, being rested, well fed, and alert will be best.
Extra curricular ... enough to show you are social, but not so much that you appear to over-extend.
AP Classes ... good if they're in the field you want to go into.
Class rank ... if it's not top handful (maybe up to 5) it won't count.
Essays ... fairly important. CHECK SPELLING AND GRAMMAR.
Volunteer work ... always a bonus, especially if related to your field. (Tutor math for other students if you want to go into math.)
Internships etc ... very helpful if you do well and work hard at them. If you have a tendency to blow it off and not show up, it's going to count against you. (You'd be amazed how many people think just having the internship, not doing well, is what counts.)