How to work out for paddling in surfing?

How do I train my shoulders for the specific motion of paddling out and into waves? I know general shoulder exercises, however I can do them all winter and feel completely unprepared come spring. Without out paying for one of those machines, does anyone know advanced techniques?
Thanks

a good one is to go out on flat days and sit on your knees, keeping yourself balanced on the center of your board. then paddle with long strides going along the coast. your shoulders and triceps will kill