yeah ... most of these have been mentioned ... but here goes ...
1) take the extra time off before applying ... i took many years off b/c i'm a post-bacc non-trad ... even so, this year has been nuts ... once the application cycle kicks it takes over your life and year (granted, i'm a typical Cali resident so i applied to 25+ schools). If you have specific EC application goals (ie get you research published) MAKE SURE and have then accomplished before applying ... do NOT plan on adding to your resume during the application year.
2) RAWK the MCAT ... take it when you are fresh on the material. if need be, postpone your test date until you are ready, make sacrifices, take time off of work, do what must be done to get your score into the kill zone ... then take it once. kill it.
3) I regret not pushing harder to get my research published earlier ... the submission/revising process can drag on and it took more time/energy than i had anticipated. According to various adcom members i spoke to, "working on submitting a paper" is VERY different from "successfully published a first-authored manuscript in XYZ medical journal." In my case, the latter might have gotten me accepted at schools where i was waitlisted/rejected.
4) get a paid clinical job instead of volunteering ... the fact that many of my clinical experiences were un-paid/volunteer meant that i could manage scheduling/life without too much difficulty ... but, as i heard from adcom folks, everyone has volunteer experience (and the accompanying volunteer coordinator LOR) whereas only some folks have "real clinical work experience" ... working means you get paid and you have true responsibility (in the eyes of the adcom) ... while many volunteer gigs are legit and have tremendous amounts of clinical exposure and responsibility, some are very limited. unfortunately, adcom's often assume that all volunteer experience is the same ... limited.
5) apply at the stroke of midnight on June 1st ... this means getting transcripts in earlier, essays and LORs ready earlier ... EVERYTHING EARLIER. This would have helped my prospects and my budget - earlier applicants get first picks of interview slots and can bunch interviews into fewer trips ... in my case, i was verified in late July, which was early enough to get multiple interviews, but i wound up flying back and forth across the country at $500 a pop ... brutal. many of the applied-june-5th kids i knew managed to bundle 15 interviews into one or two actual trips, which saved them a ton of $.
6) trim down the EC list and get deeper on the few that really matter to you. i found it hard not to try and get involved in everything, mostly because i genuinely liked doing different things, but also because it seemed like every other applicant had 20+ cool EC's. In hindsight, being more picky about where i put my time could have helped my app.
also, SDN/MDapps makes it seem like every applicant has volunteered 10,000 hours in the ER, 10,000 hours at the homeless shelter, four different summer internships at the NIH, founded and directed a pre-med group, competed in triathalons, played piano since age 6, dean's list, etc, etc. This is extreme, but the common MDapps items are indeed common. while these EC's (ER volunteering, shadowing, community service, pre-med student group) are truly great experiences (imo), the adcoms see a stack of apps each year and tend to ignore the "usual stuff" and look for the unique and different.
best of luck ya'll!