It can be a royal PITA depending on what style boat you have.
My experience with suction cups is bad. They don't work for long.
Maybe these could be useful for you. Use a soda bottle partially filled with sand to prevent freezing.
In a pinch with a big rain storm coming I sorta did what arks mentioned. I went and bought a bunch of gallon jugs of spring water and tied them to my trailer cover. Worked! Of course if my boat was slipped regularly, I'd have a proper mooring cover made.
