I do exactly as dingbat suggested. I put a full charge on my batteries before I put them to bed for the winter, stored in a garage. In the spring of the year, the batteries always still have a great charge on the enough to start the motor with no problem.
The one thing I think is a "must do" before winter, is to change the lower gear unit lube. That way, if there is any water in the oil, it will be gone and not freeze during the winter. I learned this the hard way years ago. It was an expensive repair bill on my 8 H.P trolling motor when the water in the oil froze and broke things.