MIT requires all deferred applicants to submit a midyear grade report reflecting grades from the fall semester, so make sure it reflects your best academic performance and a rigorous course load. You can also submit additional support documents to be added to your file. You can, for example, request additional letters of recommendation, or provide evidence of your talents outside academics, such as artistic or athletic skill.
Submitting additional materials can help reassure you that you a presenting the best encapsulation of yourself to the admissions committee, but you should also maintain a healthy, realistic outlook. You should come to terms with the fact that, even if you are an incredibly qualified candidate, you might not be admitted. For the 2013 incoming class, almost 70 percent of early action applicants were deferred, and only 4 percent of those deferrals were eventually admitted. Begin thinking about other schools at which you would thrive and pursue your academic goals so that you will have strong options from which to choose.