How did you feel during the test?
Were there numerous distractions, did you fall ill, did you get too nervous? If any of these scenarios happened, I would advise retaking the MCAT after you also take the following points into consideration.
How close to your average practice exam scores was your actual MCAT score?
During your preparation, you should have taken numerous practice MCATs and recorded your scores. Is your actual score significantly lower than these scores? If your actual score is significantly lower than these practice scores, around 4 points, then you should consider retaking the MCAT after re-preparing for it.
Do you feel as if after taking it, you now understand how to study for it better?
The first time I took the MCAT, I studied primarily by doing stand alone questions, not realizing that the majority of questions are passage based. If a scenario like this arises for you, consider perfecting your studying and retaking.
Are you confident you can do at least 2 points better?
If you are almost certain you can score at least 2 points better, you should consider retaking the MCAT. If you are unsure, I strongly advise being careful. You do not want to take it again and score worse than the first time!
If your stats are not up to par with the schools you are determined to get into, consider retaking the MCAT.
You should always have a back up plan. You may have to be willing to take a year off after undergrad in order to increase your MCAT score. During this time, you may be able to participate in research or do a Master's program, which will both increase your chances of getting into medical school.