If you applied early action or early decision to a university, the admission's committee reviews your file keeping in mind three possible outcomes: an acceptance, a denial or a deferral.
Because this college deferred you, this means they would like to review the regular decision pool of applicants before making a final decision. This also means your mid-year senior grades will probably weigh heavily in their decision, as well as any new test scores or new information. Some colleges suggest you send a follow-up letter detailing that new information. If a university is your first choice, you should clearly state this in the letter and promise that you will attend if admitted.
I suggest you contact your specific area admissions representative via e-mail and ask how you can strengthen your application. The letter should sound optimistic but not arrogant as you explain your excitement with this new opportunity to be reviewed further. If you skipped out on the optional interview, you might ask if an interview is still possible. Would they be willing to review a supplemental letter of recommendation? The only extra letter should be from someone who has new information to share about you that might strengthen your candidacy. Remember that sending in letters from famous alumni, members of Congress, etc. won't help unless these individuals seriously know you and your academic abilities.
Please be patient. You don't want to nag your admissions representative, and you want to show maturity about the process. The final decisions will probably not be made until late March.