i've always found running a dehumidifier in the room helps. it's amazing just how much water those things can draw out of the air! airflow is really important at stopping the growth of molds as high co2 environments encourage growth.
if I use coir, I reuse it for several grows. as long as the pH is right and it's not harboring mould or anything, it's all good.i usually give it a good few rinses though to eliminate build up of salts and so forth.