Slack off in high school. Nothing will get you a rejection letter quicker than poor grades. While you may be able to get by with a B or two, the higher level universities may be looking for people with straight As.
Take easy courses in high school to get all As. Most colleges do not only look at the grade you received at the end of the term, but what classes you took. A college will often pick a student with lower grades if they challenged themselves with Honors and AP level classes.
Avoid taking the ACTs and SATs seriously. Better yet, don't take them at all. Even if your college claims that they do not accept students based on scores for the standardized tests, they will look at them if they are available. Since not all high schools are created equal, these tests provide a way for college admissions officers to judge one student from another efficiently.
Be sloppy and careless with your admission papers. This will show that you are not a serious or careful student. Nothing looks worse than accidentally sending a sticky note addressed to or from your parents with the rest of the papers. Coffee stains, smudge marks and Whiteout all over the papers will make you appear less desirable.
Give your teachers and counselors absolutely no time to write your letter of recommendation. They have lives and other things to do. Make sure you give them ample time to write your letter. Let them know when your deadline is.
Forget to tell your teachers anything important about the colleges you are applying for if they are writing a letter of recommendation. Also, when you get the letter don't read it. Since many teachers write hundreds of letters of recommendation, they often have templates prepared where they will plug in your name. They are only human and have other things to worry about, so they might forget to change other important facts such as gender. Just send it in anyway.
Slack off on the application essay -- or put too much faith in it. It doesn't matter if your grades or the essay are outstanding, one will not cover for the other. Colleges are looking for serious students and slacking off on one will show that you are anything but serious.