Half of what goes wrong over winter is water sitting somewhere it cannot leave.
Supporting the hull
Bunks under the hard points, not under the middle of a panel. Check the trailer is level before you drop the weight onto it.
Covering
A cover has to be a roof, not a bag. Give it a ridge and a slope, and leave the ends open enough to breathe.
The trailer itself
Bearings, lights, tyre pressures. The trailer is the part that gets neglected and the part that strands you in May.