Dry Clean or wash all garments before you store them away for the season, now is the best time. Any stains left on clothing will only darken and become worse as they sit in storage. Cleaning the clothes also means that insects will be less likely to take up residence in your sweaters. Most clothing just needs to be washed normally or dry cleaned according to the manufacturer’s instructions. Taking a little time to thoroughly wash laundry before you pack it will keep your clothes safe during storage. Dry Cleaners have more capability to remove your stains, just cleaning or washing your clothes will clean them of course but doesn’t necessarily mean that out get our your stains.
Plastic storage containers can be used for storing clothes shorter term ( not for years ), although if any bug larvae are in the clothing, they will have a feast. Make sure you use new clean acid free boxes to avoid staining and insects. Even then be aware that cardboard boxes can attract bugs.  Storing your garments in the dry cleaning plastic bags is not recommended.