Rank programs even if you think you won't make it. If you end up in the scramble, programs decide on your record, your recommendations, your statement — and the fact that you contacted them.
Don't crowd where everyone else is. The best-known programs receive 400 to 800 applications for 6 to 8 positions. Less known or less competitive programs receive far fewer — your odds there are better, and they still make doctors.
Read the NRMP's own examples. The matching algorithm rewards ranking by true preference, not by guessed odds. If anything is unclear, talk it through with your mentor.