The common answer to seeing the map while flying at night is the red flashlight. Red flashlights are great because they don’t mess up your night vision. However, when it comes to reading the map, red is a poor choice.
Our VFR sectional charts use the color yellow to represent areas of significant light on the ground. At night this is a large part of your navigation information.
Unfortunately, red light washes out the yellow and you are unable to see all of that important information. This is why I usually fly at night with a white flashlight. I keep a red one on hand for tasks around the cockpit, but reading the map is the primary use for a flashlight, even in an aircraft with decent night lighting.
How do I keep from blinding myself with the light?
I use a simple technique in which I cup my hand around the front of the flashlight and only let a small shaft of light escape from between my fingers onto the map. I am careful and deliberate in handling this light.
This has allowed me to easily read the map and see all of the elements needed for navigation without any loss of night vision.