Always run the ventilation fan in the bathroom during and at least 20 minutes after
showering and bathing. If you do not have a bathroom fan I recommend having one installed. And make sure it is vented to the outside not the attic as it can cause moisture to build up in the attic.
Check to make sure the dryer vent is connected to the dryer. Make sure the vent pipe does not have any holes and is vented to the outside.
Repair weather stripping around all doors. Caulk and seal leaky window frames. Repair or replace broken window frames.
Always use a vent hood or downdraft ventilator when cooking, especially on spaghetti night or when boiling water. In order to evacuate unwanted moisture, these ventilation systems should be vented to the outside.
You can buy inexpensive, indoor thermometers with a hygrometer, which measures humidity, to monitor actual conditions within you home. You can get them at any box store or online.