The best option is not to use iron nails at all. They are not good resistors against rust and you will have to take care of them. Instead use steel nails. They are a little expensive, but are more durable and long lasting than their iron cousins.

If you must use the iron nails, then either coat them with a thin film of moisture resistant paint or coat them with nickel. Paint is the better option as it is easier and cheaper though. Alloying is also an effective method of rust prevention and chips and scratches in the surface are generally not important.