Your basement is the part of your home must vulnerable to excess moisture. Left untreated, neglected basements can serve as a dangerous haven for mold and mildew, which can have extremely deleterious effects on the health of you and your family.
Proper basement maintenance begins outside. Check to make sure that your gutters and spouts direct water away from your home so it does not leak through too near the basement. The soil grade around your house should also direct water away from the foundation – that is, the ground around your home should be very slightly sloped so water runs away from the walls.
A dehumidifier can be an easy way to remove excess moisture from the air, and is one of the best ways to defend against mildew and the unpleasant smell that comes with it. For an entire basement, you may have to purchase several individual units: be sure to check the manufacturer specifications on how large an area a single humidifier is rated for, and adjust your purchasing decisions appropriately.
A special sealant and paint around the walls of your basement can serve as an extra layer of protection against moisture seeping in through minor gaps not easily seen or closed (though obviously it is ineffective against large cracks and gaps). It also helps prevent condensation on the walls of the basement, which evaporates and leads to high humidity in the area.
With changing temperatures over several years along with slightly changing soil composition and moisture outside, it is normal for the concrete of a foundation and of basement walls and floors to develop cracks: the partitions expand and contract and settle in slightly different areas than where they were originally poured.
The longer a crack is left untended, the more moisture seeps into the basement, and the more difficult and expensive it becomes to fix the crack later on. Some home-use sealants are adequate for small cracks and can be done at little cost by the homeowner. In extreme situations, you may need to contact a professional to help diagnose the problem and prescribe a long-term solution.
If the area surrounding your basement is particularly moisture-prone and poorly drained, a sump pump may be a good option. They can be expensive to install, but are very effective. Water is funneled into certain basins before being pumped out away from the house. They do require power, and in situations such as heavy storms where they get a lot of work, a backup generator is strongly suggested to make sure your basement does not get flooded due to a non-functioning sump pump.
Other professional drainage options exist beyond sump pumps: a French perimeter drain system around the foundation is an unpowered alternative and may be effective enough for your home. They may also suggest sealing your basement from the outside, on top of any additional sealing added to the interior. Basement waterproofing companies have a number of options depending on the severity of the problem and the home in question.
Finally, other systems in your home may add unnecessary moisture to the basement, repairing of which may solve the problem entirely. Small plumbing leaks can, over time, add a tremendous amount of moisture to a basement, so certifying that all plumbing is fully operational is a good early step. If your clothes drying machine is in the basement, be sure that it is vented properly to the outside and there are no perforations in the venting system.