What’s long exposure
Long exposure photography means using a long shutter speed, a tripod, natural density filter up to 10 stops and often a remote shutter release. The resulting effect is to sharply capture stationary elements of a scene, while blurring, smearing, or obscuring any moving elements. It’s a beautiful effect, and can lead to incredibly beautiful images.

Clean your sensor
Make sure you clean your sensor before going out. With the use of small apertures (see further), you'll all sensor spots. Without cleaning there will be a lot of Photoshop post-processing to clean up the photo.

Check your batteries
We're talking about long exposures that can take seconds, minutes of even an hour. I shot photos with 30 minutes exposure. So charged batteries are a must and spare batteries as well.

Go Steady, Carry your tripod
Whenever you’re leaving your camera shutter open for a long period (anything above the reciprocal of the lens’ focal length – i.e. 1/50th of a second for a 50mm lens), you’ll need to have something to steady your camera. Holding it by hand is perfectly fine for many shots, but not if you’re wanting to experiment with a long exposure.
Start by picking up a tripod. It doesn’t need to be a particularly fancy or expensive model, particularly if you’re only just starting out. Choose a size that fits your compromise between being large enough to be useful, but small enough to carry around as needed.
You could also purchase a cable release, to avoid any shake when you physically press the shutter. A cheaper alternative to this is simply using the self-timer on your SLR. Set it to two seconds, and then stand back!

Use a small aperture.
If you need to, or just want to, lengthen the shutter speed of your long exposure, start with the lowest ISO available on your camera and then use the smallest aperture available on your lens. This has the added benefit of creating more depth of field, likely a sharper picture, as well as star effects from pinpoint light sources when the aperture is smaller than ƒ/16 or ƒ/22.

Grab your filters, Use a Neutral Density Filter
Filters are sheets of glass or resin that you put in front of your lens to alter the light hitting the sensor. They come in two formats: round filters are meant to be threaded on to the front of the lens, while square or rectangular ones must be used in conjunction with a filter holder system, such as that manufactured by Lee.
If you’re interested in landscape photography, there’re a few filters that I can’t urge you enough to invest in.

Kind of Neutral density filter
ND filters are dark pieces of glass or resin that reduce the amount of light entering the lens, and come in various strengths. Different manufacturers use different nomenclature, but as a general rule, .3ND results in 1-stop light reduction; .6ND is 2-stop; .9ND is 3-stop, and so on. For creative effects, you can’t beat a 10-stop ND, but keep in mind that this reduces light so much that it’s extremely difficult to focus with this filter on. If there’s a slight breeze, you can extend the shutter time to capture clouds as streaks, as in the following photo. With an interesting foreground (which is severely lacking in this example), you can turn an ordinary photo into a very interesting one with an ND filter. You may be able to achieve the same effect with Photoshop layers, but it’s going to be a lot more work than getting it right in the field.

Graduated neutral density filter
A good set of GND filters is an absolute necessity for landscape photography. Unlike ND filters – which block light everywhere in the frame – GND filters block light selectively. There’re three types of GND: soft-edge, hard-edge, and reverse.
Soft-edge GND have gradations that slowly transition to clear, and should be used where the horizon isn’t well-defined. Hard-edge GND has more abrupt transitions to clear resin, and is your best bet when the horizon is well-defined. Reverse GND are more expensive, but there’s a good reason to use them: when the sun is near the horizon, you want to block its light the most. The sky higher up the frame is already a bit dark, so don’t want to darken it even more.
As with ND filters, GND come in varying strengths, but the most common ones are 1, 2, and 3-stops.
As an alternate to these expensive filters, you can take multiple photos, with different exposures for the foreground and background, and merge them in Photoshop or Photomatix, but this won’t always work. For example, if it’s a windy day, clouds, leaves, and waves will have traveled between your photos, and merging them will be a nightmare. On the other hand, there’re many cases where you must use exposure blending instead of GND filters – for example, when the horizon isn’t well-defined: you don’t want the top half of the tree to be darker than the lower half.

Circular polarizer
I’ll have to take some with-filter and without-filter photos to show you the effect, but in essence, a circular polarizer cuts glare and enhances colors and contrast. Use it on an overcast day to reduce the ugly gray sheen on leaves and flowers; use it on sunny days to make clouds pop, and darken the sky; use it in the fall to make your reds more vibrant by cutting reflections (this isn’t the same as increasing saturation in post-processing – it’s about removing unwanted reflection from objects). Of all the filters discussed here, this is the only one that you can’t replicate in Photoshop.
This filter is usually round, and has a movable front that you adjust to select varying amounts of polarization. The effect is most pronounced when your camera is pointing 90 degrees away from the sun; a word of caution though: wide angle lenses cause the sky to be darkened in varying degrees, so you might not want to rotate the filter to its full extent.

Filter systems
You can buy either round filters that you can attach to the front of your lens, or you can buy rectangular ones. A word of caution: before you head out and spend a fortune on filters, check if the front element of your lens rotates when focusing. If it does, using filters will be a much more difficult task than if your lens has an internal focusing mechanism. The problem with lenses where the front rotates during focusing is that if you’ve attached a filter, it’ll move when you’re focusing, and if you’ve already spent quite a bit of time aligning the filter properly, you’ll need quite a bit of patience to set it again after focusing, and then focus again! This problem crops up often on cheaper lenses, so before you buy new lenses, always use your favorite search engine to determine if the model you’re looking at has a rotating front element.
That said, I recommend that you get a round circular polarizer (check your lens documentation to see what size you need) that you’ll thread to the front of the lens, but you must get a rectangular GND. Here’s why: you saw above that a GND is half dark, and half clear. If you get a round GND the size of your lens, you’ll be forced to take photos where the horizon is smack in the middle of the frame, which will make for a horrible composition! Rectangular filters allow you more leeway as far as composition is concerned.

To use rectangular filters, you need a mechanism to attach them to the lens. I use the Lee Foundation Kit in conjunction with Lee 4″x6″ rectangular GND filters, and 4″x4″ ND filters. For circular polarizers, you can’t go wrong with B+W. There’re many other brands available, both less and more expensive than what I use, and ultimately your budget will define what you should get.
To use the Lee filter system, you attach a ring to the front of the lens; to this gets attached the actual filter holder. You then slide in the GND or ND filter from the top. While looking through the viewfinder, I usually move a horizontally-aligned finger up and down to determine exactly where the transition from dark to clear is, and then adjust the filter accordingly

Head Out at Night
Although there’s plenty to be captured in the day, long exposures at night take on a completely new feel. It’s a widely showcased type of photography, but one that never fails to impress and amaze. It shows a blurred aspect of light that we never see with our own eyes, and captivates viewers for that very reason.
The usual advice applies – You need a tripod, lots of patience, and a few interesting sources of artificial light to incorporate in the photo!

Use the Bulb Mode
Set your camera to bulb mode. Most cameras have a maximum exposure of 30 seconds in other modes. With the bulb mode you can go beyond 30 seconds and choose your own exposure length.

Noise in pictures
Even at low ISO, super long exposures can introduce noise in the form of hot pixels. You may not be able to see these when viewing the results on the LCD screen of your camera, but when viewed at 100 % on your computer monitor; you may find a number of bright red/green/blue pixels in your image.

Shoot RAW
An effective way to remove them is to take an exposure of identical length, at the same ISO, with the lens cap on. The hot pixels will be identical in all shots, almost like a finger print of your sensor, so by replicating the exposure with the lens cap on, you will generate an entirely black image, with the same hot pixels, to subtract away from your chosen image during post-processing.
Some brands of filter are known to leave more of a colour cast on the final image. This is a great reason (one of many) to shoot in RAW, as the colour casts can often be corrected during post-processing. Sometimes, for particularly long exposures, it may just irreversibly compromise an image and, in those instances, a black and white conversion is often the best way to overcome it.

Enjoy the view
During the exposure you have all the time to enjoy the view and explore the surroundings for next exposures or just relax looking at the panorama.

This article was written from others reference found over Internet.
Very cool example of LE for inspirations
photo.tutsplus.com/articles/inspiration/70-dazzling-long-exposure-photography-examples-tips