@viphan, damn I wish we had a guide like this. Hopefully we can this stickied by a moderator.
Most of what I will say, echoes the views of the previous posters, but what I can say is that taking a gap year has truly been a transformative experience, but it wasn’t without its challenges. I took my gap year due to financial reasons and in that time, I traveled with a group of doctors to Haiti for a few months, then I tutored for a while in the US. Finally I was fortunate enough to get a research assistant position at Columbia and NYU. Nothing except the trip was planned and for the longest while I didn’t know what to do with my time, so planning is key.
Quick side note for those looking to go on a gap year just to reapply to top schools, keep in mind that they’ll expect you to make the most of your time off. So watching tv or doing things that could have easily been done while in school isn’t really impressive or worthy of taking time off. So just make sure you have a well thought out plan for your year. There are several programs you can do for free like those by Americorps.