Mold is extremely hazardous to your health. It spreads through tiny spores that can be breathed in and cause serious illnesses. All mold needs moisture to grow so if you can control the moisture in your home you can control the mold.

Dry up any wet areas as soon as possible.
It’s always easiest to clean up immediately anyway and if left for over 24-48 hours it is much more likely for mold to start growing.
Make sure your house has proper airflow.
Stagnant air allows moisture to stick around and helps mold begin to grow. With air constantly moving it is much more difficult for mold to even start to grow. Pay special attention particularly to bathrooms and other places of high humidity need to have proper ventilation.
Take care of your roof.
Debris left on your roof for an extended period of time increases the potential for problems. Clean it off as soon as possible. If you do find that you have a leak you need to repair it immediately. Leaks in your roof can escalate to bigger problems quickly. Another part of the roof that is often overlooked is making sure your gutters are clean.
Help water run AWAY from the foundation of your home.
Along with keeping your gutters clean you need to make sure that the water going through them is actually guided away from the foundation. The ground around your home should slant away from your house to help with this. If water is pooling around your foundation take the time to build up those areas with fill dirt.
Use dehumidifiers in problem areas.
Some places in your home may be more difficult to get proper airflow. In these cases you may consider purchasing a dehumidifier.
Dry your clothes immediately after washing them.
Damp clothing left out is a great place for mold to start growing. You may also want to leave the door to the washer open after using it to help dry out in the inside and prevent mold from growing in your washer.
Clean your home regularly, giving special attention to bathrooms and other humid areas.
Simply cleaning your home regularly greatly reduces the chance of mold forming in your home. As always the problem areas often need a little extra work.
Let some sunlight in.
Mold likes dark damp areas. So naturally a little sunlight can go a long way in mold prevention.