Indoor storage on the side of the hull on padded blocks is always preferred over outdoor storage, but if that is not possible, try to store the boat deck side down with a good poly tarp that covers the entire hull and trailer. Consider jacking the trailer up and setting the axles on blocks so the tires are not in contact with the ground to prevent tire rot. Before storing, make sure all water is out of the inside of the hull by turning the hull on its side and unscrewing the drain plug on the starboard side of the deck. If water freezes inside the hull, you risk cracks and leaks next spring.
Others may also want to chime in with suggestions.
I've done both. While indoors is preferable, I've found that with a little extra care, outdoor storage can be almost as good. For the fifteen years I stored my fish on inside on its side. Then I moved into a house with no garage. I've stored it right side up on a trailer under a tarp ever since. It's been 16 years outside none the worse for wear. I'd recommend storing rudder, daggerboard and sails indoors if at all possible, even if you have to take it off the spars. It turns out that insects think that Sunfish sails make lovely winter homes, while the wood in the rudder and daggerboard tends to rot due to dampness. I raise the trailer wheels off the ground and I make sure that they are under the tarp as well, because I find that Winter UV Exposure and the freeze-thaw cycles in my muddy back yard shorten the life of the tires.