Open as seldom as possible (ditto to above), especially if it's front opening.
Try to get food
already chilled down so the work
the boats fridge has to do is reduced.
If you can, freeze gallon jugs of water to take onboard (blocks of ice that you can drink as they melt and less messy).
If it's a chest type and has a drain hole in the bottom, make sure the hole is plugged. Cold air drains out just like water does.