From my casual observations of the photographic habits of others around the world I would suggest that one of the most common mistakes most people make concerns taking photographs in low light.
Whilst lighting conditions remains one of the trickiest aspects of photography to wrestle with, low light photography shouldn’t be an impossible task for novices. I’m sure professionals have much better techniques to deal with low light but these steps are catered for the novice. I’ve over simplified the processes here to better explain. I must stress that I’m not a professional but I have some experience at amateur photography. These tips should be regarded with those things in mind.
1) The basic problem with low light is camera shake, which results in blurred images. The less light there is, the longer the shutter speed required. So the easiest first step is to increase your shutter speed. A very rough rule of thumb is that 1/30 sec is average minimum shutter speed that most people can hand hold a camera without camera shake. Alternatively you can increase the aperture (lower f stop). Larger apertures frequently require less light and hence faster shutter speeds.
2) Increase your ISO. Once upon a time one had to buy ISO specific film. That was a nightmare when the weather conditions changed. These days, a lot of cameras this is just a setting on your camera. The ideal remains 100 ISO, which will give you the least amount of grain but not always possible. Just keep increasing the ISO until you arrive at a comfortable shutter speed. With experience you may be able to hand hold at slower shutter speeds. The image below was taken with a high ISO and hand held at 1/10 sec (which is very low and not recommended for novices).
These images below were taken with high ISOs and more acceptable shutter speeds. There is some grain present but not too bad.
3) Invest in a tripod. This is by far the best option for when the light is too low for steps 1 and 2, if not the most convenient. Using a tripod you should be able to use whatever settings you choose without fear of camera shake for the most part. Using a shutter release cord will also help. Some of the flimsier tripods may still shake in windy conditions. The image below was taken at 100 ISO with a tripod and a shutter speed of 1 sec. The grain is minimal and it is as sharp as the medium allows.
4) Flash photography should be avoided if possible. The results are always harsh and unflattering. Flashes should only be used if you have no other option. One of the most common mistakes is attempting to take images of subjects way too far away for the flash to be effective. I shake my head every time I see flashes going off during sporting events. The average built in flash has a reach of only about 5 metres. All you would be doing is lighting the back of someone’s head. The image below was taken with a flash. It served my purposes at the time, but notice the harsh shadows.
Even after the image is taken one can often improve images if adept enough at Photoshop (or other similar programs). One last thing to consider is switching to B&W mode when light is low. This won’t affect the shutter speed but may result in a more appealing image. Quite often when days are overcast and gloomy the images will appear dull and lifeless. You will find that B&W mode a more forgiving format. If you compare the images below, the full colour version is dull and flat. The B&W version however, has more of an artistic look (IMO anyhow). Whilst I converted the image to B&W in post, a lot of cameras will let you switch modes before you take the shot. I don’t think it makes any difference. From my standpoint it’s more convenient to shoot in colour first and decide on conversion later. With more experience at Photoshop you could even make the B&W version into a duotone or tritone image, but that’s another story.