Yes, it's yet another collection of all the things that I wish I had known prior to applying for colleges. But hey, you're already here, so just keep reading...

• Start narrowing down your list of possible colleges in your junior year. By the time I had to fill out applications I was down to two choices, so I didn't do nearly as much paperwork as most people. Three or four is a good number, five or six is a lot of work, seven or more is insane.
• Take the SAT near the end of your junior year at the latest. This will leave you enough time to retake it if you forget to bubble in your answers or leave your brain at home that day. Seriously, the SAT is not something to worry about. You should be able to get at least your PSAT score with the same amount of effort.
• Take the SAT II's at the end of your junior year (if you've already taken the SAT) or the beginning of your senior year. Check with the colleges you're applying to to see which ones to take (generally Writing, Math IIC, and a science). Don't check your scores before releasing them. I don't think anybody ever actually looks at these scores anyway, just that you took the tests and that they were consistent with your other standardized test scores. If you have a conflict with the test date (e.g. ARML), tell your counselor to arrange for an alternate date (yes, this can be done, you just have to ask).
• Find out what colleges give credit for the AP's you want to take before you spend $72 for nothing. Thus if you are considering MIT you would not want to take the Computer Science A since you would not get any credit for it, but other schools might offer credit and/or placement for the same test.
• Don't pay for anything from the College Board that is not essential. I think their whole operation is just one huge scam.
• Apply to colleges at their first non-binding date. For MIT this is called early, for Rice it is called interim. This allows you to laugh at your friends when they are frantically finishing their applications while you are holding a letter of acceptance.
• Talk to people in the admissions office so that they will learn your name (hey, it can't hurt as long as you don't make them mad).
• Get everything in well before the deadline. If you wait until midnight on the due date to mail your application then somebody will bomb the post office at 11:59. -Murphy's Law
• Ask for the teacher recommendations way in advance. Your teachers will really appreciate this and thus write better recommendations.
• Get your transcripts as soon as possible, and get one just for yourself. When you go to request one that you need the same day the registrar will be sick.
• Check with the colleges to make sure that they have received everything. MIT routinely loses stuff (you can send them two pieces of paper in the same envelope and they will tell you that they only got one of them), so you should keep copies of everything (including teacher evaluations, which is one of the things they lost from me). On a similar note, learn the location of the nearest Kinkos (trust me on this one).
• Try to be the first person to call the interviewer in your area. They are really impressed by your bravery.
• If you know somebody (student or faculty) at a college you're applying to, ask them if they can pull any strings for you (not likely, but again it can't hurt).
• Don't count on winning lots of scholarship money from Coca Cola, Tylenol, Westinghouse, etc. By all means apply for all of them, but keep in mind that unless you have a visible halo you have a better chance of winning the lottery.
• Do not pay someone to give you a list of scholarships you are eligible for. They are called con artists. You will get something in the mail saying that a recent change has made it possible for them to consider you and that you must call for an interview within the next few days. Just throw it away. Your counselor should be your main source of information, along with reputable websites (e.g. Fastweb).
• Hear your parents out, but do not let them make major decisions for you. When they refuse to pay for the college of your choice, ask them what they would pay for and offer to be responsible for the difference (i.e. jobs and loans). They do not think rationally at this point, so don't try to argue more than necessary.
• Get all the financial aid stuff done asap. FAFSA processing is very slow when the government is shut down, and colleges won't offer you a penny until your FAFSA has been processed.

• Join clubs run for offices. Not too many, but show that you didn't spend _all_ your time studying, you can be committed to something, and you have some leadership skills.
• Play a musical instrument in a school group and/or outside of school.
• Play a sport on a school team and/or outside of school.
• Go to competitions of all sorts (academics, fine arts, and sports). This shows that you aren't doing these things just to pad your resume. :)
• Do community service, the scholarship people love it (it also makes you feel good, try it!).
• Do not value your class rank over extracurricular activities (within reason) unless you have some reason to be valedictorian (e.g. scholarship, insane parents, etc.).
• Take advanced classes. Only a very small percentage of high school students have taken post-calculus classes.
• Try to get your name mentioned in a widely distributed publication. People are really impressed by this.
• Develop an unusual talent with which you can impress interviewers.

It should be noted that this advice is intended mainly for students planning to attend highly selective colleges. If you are going to a public college or are struggling to get a 4 digit SAT score then I would not advise taking all of this advice (e.g. I would place more emphasis on improving test scores). I take no responsibility for anything bad that might happen as a result of people following my advice, but I will take full credit for any good that results. :)