Visible signs of mold. Clean surfaces where there is mold with bleach. Fix all sources of leaks. Remember, mold thrives in damp conditions. Check houseplant soil, make sure it is not always very damp.
Mold loves moist soil.
Keep firewood outside. Pay special attention to the basement, which is the part of your home mold is most likely to be a problem. Keep it as dry as possible. You may want to keep an air purifier in your basement. Clean the shower floor frequently to ensure that mold does not accumulate.