Tips and advice:
Excel in your postbacc program. This is the time to make your application strong. If your weakness is grades, now is the time to prove you can succeed in medical school by excelling in your science classes. You may need to cut out unnecessary activities, work, volunteering, etc. If you cannot do this, you may want to consider doing those things first (working and saving money, finishing a project, etc.) and then starting a postbacc program.
Most postbacc programs are affiliated with medical schools, so get to know medical students, the Dean of Admissions and Program Directors before you apply to the program. They can be invaluable resources, and give you great feedback.
Know your professors/advisors and start asking for strong letters of recommendation. Mediocre letters are not advised. Talk to people who know you well and are in a position to evaluate your work. See specific school guidelines about who can or cannot write your letter. (Do not ask for letters from high school coaches, clergy, family members, psychologists, etc. There are some exceptions, so consult a mentor.)
If you have not taken your MCAT, or need to re-take your MCAT, talk to your advisor and ask the postbacc program if there is MCAT tutoring/prep courses available and when the best time would be to take it.
Start drafting a personal statement early. Look for resources within your program or school, at LMSA events, or through other services that may help you revise and perfect your personal statement. Outline what you want to address in it, and start free-writing. Please revise, revise, revise, and have various people read it (not just your mother and your best friend, but people who can critique it from an outside perspective).
DO MOCK INTERVIEWS! This is really critical. You’ve worked so hard and come so far and once you get your foot in the door to get that interview, this is it: your ONE shot! You must be prepared, professional, and polite. Look for conferences, LMSA events, and professors/pre-med advisors for mock interview opportunities. Ask for critical feedback.
If you have not done so already, read about what is going on in the world and become informed about social issues, especially, but not exclusively related to healthcare. This will help you on your interview. Medical schools are looking for real people with compassion, insight and real world experience who will bring that breadth of knowledge and understanding to the classroom and the clinic.