It's always tempting to use a flash to take pictures easily in situtations in which you wouldn't otherwise be able to take a picture. If at all possible, avoid using your flash, however:

• Flashes have a harsh light that will show unflattering wrinkes and blemishes.
• If your subject is close to a wall, your flash will leave a sharp and prominent shadow right behind your subject.
• Flashes have a bluish light that can color your photo unless you're using the right film or have the color balance set properly on your digital camera.
• Flash light, being from a single point source, is unlike all of the other light you see in the world, which is more or less diffuse. This will also tend to highlight unflattering details.