There's a very useful five-page guide to applying business school from Harvard's Career and Academic Resource Center (CARC) available at:
http://www.extension.harvard.edu/sites/default/files/ext_carc_bus.pdf

It's aimed at students in the faculty and arts and sciences, who might be applying for any business school, and there's nothing particular to applying at Harvard Business School.

A lot of good and broadly relevant info here. A few comments:

The PDF advises candidates to do informational interviews with people in the types of positions you'd like to be in. Ask them about how an MBA is viewed in the particular industry, the applicability of the degree, and even preferred schools. This is good advice - and you can use this information to determine school choice and even elements of your application essays. To find relevant people, use LinkedIn or your own personal network.

Preparing for business school: "Business schools tend to focus on impact more than scale." A great GMAT and GPA should be rounded out with extra curriculars or volunteer experiences. If you have four years of work experience in IT, try to have something more you can point to - this separates you from your competition.

Age: PDF says the average age of b-school students is 28. While ages vary by school and by region, use 28 as a gauge. If you're substantially older or younger, you'll have to make a solid case as to why you need an MBA at this point in your life.