If your startup has at least one founder who is a currently enrolled MIT student, you can come to the Martin Trust Center for MIT Entrepreneurship for coaching support. The Trust Center provides coaching to students as the need arises, stocks ramen noodles and serves free coffee, and is also home to a lot of resources available to enrolled students at MIT. Students may email firstname.lastname@example.org to request a meeting and their request is typically met within a couple of days. Any student with startup related questions may request a meeting - they do not need to be with a previously formed startup team.
If you have no current students but at least one founder is affiliated with MIT, and you are in a previously formed team that is committed to building a new venture together, you can try the MIT Venture Mentorship Service (VMS) which serves all of the MIT community. This is a structured mentorship program in which VMS will assign a team of highly accomplished mentors, who will meet with the startup team once every few weeks over a period of several months (or longer if it makes sense) to help them build and grow their business over time.
Note: Current students may also work with VMS at the same time they work with the Trust Center - the coaching and mentoring that they get from either experience is very complementary.
There are also many programs at MIT that provides mentorship as a part of their offerings. MIT Sandbox, DesignX, Deshpande Center all support programs that connect students to mentors as a part of their benefits.
If your startup does not have any affiliation with MIT, you can leverage the MIT Enterprise Forum of Cambridge, which serves the entire startup community in the area. The MIT Enterprise Forum runs a Mentor Smart program twice a year – applications are due by October 2, 2015 for the Fall 2015 cohort.