Human beings cannot see in pitch darkness. In general, they do not have very good night vision. However, it is possible to improve your ability to see in the dark.

[edit] Steps

Peripheral vision is more night sensitive.

Take advantage of the structure of your eyes. The human eye has 'rod' cells and 'cone' cells on the retina, which is the sensory layer at the back of the eye. Rod cells and cone cells are distributed evenly throughout the retina except for the fovea, which is a small area on the back of the eye directly opposite the pupil. At the fovea, there are only cone cells. This is an important thing to know because the 'cone' cells are more proficient at color detection, whereas 'rod' cells are better for low light and detecting movement. Therefore, when trying to see in low light, try not to look directly at the places you are trying to see. By using your peripheral vision you are using more rod cells, which work much better in low light. This takes a great deal of practice for most people.
Keep your eyes adjusted for the dark. If you're in a lighted area and know you're going to be going into a dark area, close your eyes tightly, or at least squint your eyelids before entering the dark to give your eyes a chance to adjust. If you can't close both eyes, close one or place a hand over one. This works well when driving into tunnels. Once you're in, avoid looking directly at any light source, no matter how dim you think it is. It takes longer to adjust back for the dark than it does for your eyes to adjust for the light you just looked at.
Practice. This can be as simple as shutting out the lights in a room and closing all portals, allowing only the ambient light that slips in under the door.
Scan, don't stare. If you look at something, or a place, in the dark for too long, your eyes will become less sensitive to what little light there is. If you scan your eyes back and forth over the area you are looking at, you will be using different areas of "rods" as described above, and you will be able to see details clearer.
Protect your night vision. If you do need to use a light, having a coloured lens over the light will help preserve your night vision. White light, containing all frequencies, from red all the way to violet (the visible light spectrum), will ruin your night vision the fastest. Red is favored when you need to recover quickly, green or blue-green should be used for acuity. Whatever light that is used, it's important to use as low intensity as possible.

[edit] Tips

In the martial art Ninjutsu, students are told in order to see better in the dark, get lower than what you are trying to see. This may work by silhouetting objects against light from the moon and stars.
For best vision, allow plenty of time to adjust to seeing in the dark. It typically takes about 20 to 30 minutes for your eyes to fully adjust to dark conditions ^[1]
Be aware that nicotine may diminish your ability to see in the dark^[2]. It may be a good idea to avoid cigarettes, even second-hand smoke, when you need to see in the dark.
Taking Vitamin A supplementaly can help increase or retain an eyes photosensitivity.
Avoid looking directly at light sources while navigating in the dark. Even if you aren't looking at them directly, light sources close to your face will diminish night vision. For example, if you are smoking, the red/orange glow in front of you provides enough light to cause your pupils to contract, therefore killing your night vision. Similarly, you will not see much if you're wearing a miner's helmet with a light or carrying a flashlight (excluding what's illuminated by the beam of light itself, of course).
Special Forces use the technique of squeezing their eyes shut tightly for ten seconds - once they are in the dark. While the technique can seem effective, scientific studies have not proved it works. This may be a case where the brain and belief override normal physical reactions.
In World War II, Soviet Specops used this technique: eating a piece of sugar some time before going out in the dark and lighting their open eyes with a red flashlight for approximately 10 seconds.
Different people have different levels of ability in night vision. While these techniques can help you reach your full potential, your full potential may not be as great as that of others.
Look through binoculars. They will gather and intensify available light.
Look for shapes, not colors. In tall grass, look for horizontal lines that stand out against the vertical grass. These will indicate an obstacle/target.
When moving around in the dark, it is safer to move slowly and slide your feet, using your toes to sense obstacles in front of you. You're most stable when you have both feet on the ground, and are less likely to trip and sprain, or even worse, break something. Also, if you put your arms out to feel where you're going, cross them at your wrists and form a circle with your elbows out to the sides, thus helping you to keep your balance while avoiding walking into a post, tree trunk or edge of an open door.
An illuminated watch dial can be a handy source of very-low-power light.
Driving at night presents special challenges, especially in very dark places like mountain roads. Headlights from oncoming cars can really degrade your night vision. Look at the white line on your side of the road to maintain a safe course without having to stare into some one's high beams.
The fashionable eye patch worn by the pirates weren't always due to the loss of an eye. In fact, many pirates did it so that the one eye was good at seeing in the dark.^[3]
Stargazing programs often have an option to make the screen red, because red does not affect your rod cells. So, you can look at red light forever and still see in the dark well.
There's an urban legend about carrots helping one to see in the dark. This was the information extracted by German soldiers from captured Allied pilots back in World War II, to hide their use of new British radar technology. There is some truth to this legend. The protein rhodopsin found in the eye breaks down into retinal and opsin when exposed to light and reforms when back in the dark. Retinal is made up of vitamin A which is found in carrots and many other foods. A severe lack of vitamin A in the diet may result in night blindness^[4], but extra vitamin A will not help one see better than normal.

[edit] Warnings

Traipsing around in your home or outside the house can be very dangerous. You should weigh the facts of being "cool" walking around in pitch black against hurting yourself badly.
Be careful when practicing moving about in the dark. If you fall and hurt yourself, Don't panic! Take a few deep breaths and re-orient yourself before checking the extent of your injury and finding a light source.
Do not cross a river when it is dark, no matter how well you can see it, unless your life depends on it.
Be careful when closing one eye while driving, as this will cause a loss in depth perception.
Don't do anything foolhardy! (ie: use common sense)