Anna Scott,
Granted we that live in northern latititudes share the commonality of shorter training seasons, but I consider that to be more specifically related to water work. I consider each day of the year a "training day" and there are always a multitude of things to work on.
For most of us that work with advancing level gundogs or those involved in competition with their dogs, obedience should be considered a way of life for such dogs. Foundational to all advanced training, obedience is critical and it must be both well honed and maintained. As well, winter is a time that like us, our dogs can suffer with regard to conditioning for lack of regular activity levels maintained during more moderate temperatures. Self discipline and motivation on the part of the trainer is an important factor in maintaining standards throughout winter months.
I try to incorporate getting out on long walks in the woodlands during which times I will bring a bird bag of bumpers along to run impromptu set ups that keep a retriever engaged and motivated. Even what one may consider rudimentary for a more advanced level dog will result in cleaner delivery to hand, better whistle stops, crisper casting, etc. I often do long memory blinds which require lots of extension while maintaining long lines.
I work TRAD on both upland and waterfowl and he gets lots of experiential training on wild birds throughout the hunting season. Again while doing our walks, we work on quartering and maintaining him not ranging outside of the range parameters I want him working at when hunting upland birds.
Just a few of many things I work on when the water is hard as a rock.
Cheers,
Irishwhistler