A cushion under you, if you have a heated one even better, mittens with flip off fingers, a thermos of some hot liquid, soup, Chocolate, coffee, tea.
Stay dry, dress in layers, but not all cotton, once wet it stays wet. A hat always, If you don't have an enclosure, design one out of a tarp you can use it as a wind block, anchor up at the bow and the boat will turn with the wind, this alone can add 10 F.
If you go out for longer periods and have a bank you can pull up on bring a pot and cook some dish or hearty soup. Done all these and they work great, been fishing with ice floats and stayed comfortable for hours, I do tolerate cold well though.