Grab your camera, your tripod, a jacket and if you're feeling saucy, a neutral density (ND) filter and remote release; let's make some long exposures!
Long exposures don't tend to result in a 'realistic' photograph unless you are shooting at night, but instead allow you to play with motion, time and light interesting ways; clouds blur, lights and stars turn to trails, people disappear and water appears smooth as your exposure time ticks from fractions of a second to seconds and minutes.
To make this type of shot work you are going to need to prevent the camera from moving for as long as the shutter is open; any small movement will ruin the sharpness of the shot. A nice stable tripod is ideal but if you don't have one you can sit your camera on a flat wall or on a bunched up jacket, you can even brace the camera against a wall or post for a few seconds with the right technique.
Now you have a stable location for your camera on the tripod you need to trigger it without causing unintended movement. Ideally you should use a remote release to trigger the shutter, if you don't have one then use the camera's built in timer to trigger the shot, even the movement from pressing the shutter button is enough to shake the camera. If your camera is an SLR lock the mirror, if it has noise reduction switch it on and if it has built in image stabilisation switch it off.
Your next issue to contend with is focusing, if it's a dark scene you may struggle to focus due to lack of contrast and if you're using a strong ND filter you might not be able to use the camera's autofocus; there are a few techniques to get around these issues. Whatever technique you use, once you have the focus you want for the image you should switch your camera to manual focus and watch you don't knock it as you handle the camera.
If you are trying to take pictures of the stars or moon you will want to focus to infinity, do do this you either need to focus on a distant light or manually focus and take a test shot to see that the stars are in sharp focus.
If it's dark you might need to focus manually but if there is still some daylight and you are using a ND filter you can focus without the filter, switch to manual focus and then drop / screw in your filter into position.
A powerful torch in another useful aid to focusing, you can light up a section a building to aid focus and also use it for light painting during your exposure as seen in the image of Airlie Monument below.
This photo of Airlie Monument was actually 180 30 second long exposures layered together in a special bit of software to create the star trails, a similar effect could be achieved by taking one long exposure but the sensor noise would become more prominent.
How long you want to make your exposure comes down to what you are trying to achieve and the limitations of your camera, but whatever you do you will find new creative ways of taking pictures and see things in a different way.
Now get out there and report back with your long exposures!