I would say keeping it dry would be the top of the list. Freeze and thaw cycles on wet fiberglass has to be hard on it. Polyester (and vinyl-ester) resins are not "waterproof" and if left submerged, will eventually allow water to permeate - so a wet freeze can be a big deal. Even just leaving a little bit of water inside the hull dramatically increases the humidity inside the hull leading to constant moisture exposure from the inside. Keep 'em dry and fix any soft spots immediately (because they'll migrate and quickly deteriorate).