Clean the filter thoroughly at least once a month with a specialized hot tub filter cleaner chemical. This goes beyond the inspection and rinse that should be carried out weekly.
Cleaning the cover should include a wipe down of both the outside and inside surfaces. A dilluted bleach solution will help protect against mold and mildew and eliminate musty odours.
If you fall into the category of having problems with calcium hardness issues you can increase levels with calcium chloride or replace some water with fresh water to lower levels.