I have always felt the winter months are best used for practicing putting and the stroke. There are many very good putting surfaces available for inside practice. I also think it is a good time of year to make a grip or stroke change without worrying about the impact that change will have on your score.
Off Season is also the best time to set up a good fitness schedule tailored for Golf Strength and flexibility. There is plenty of good information out there for golfers and you can start the Spring in great shape for improving your overall game.