Photo by David Clapp - www.davidclapp.co.uk
Use a support
If you're using long shutter speeds you'll need to find a way to support your camera if you want sharp shots. Tripod's are good but you can use beanbags, monopods or even a near by wall.
If you're shooting on a windy day or are working by a busy road where there's lots of traffic keep an eye on your tripod as the vibrations from the vehicles or a sudden gust of wind can wobble it which will result in blurred shots.
Use a remote release / self-timer
To minimise shake further, use a cable release to start your exposures. If you don't have a cable release, use your camera's self-timer so when you do press the shutter button, it's not in the same instance you take the shot, preventing shake creeping into the image.
By using a fast lens, such as the 50mm f/1.4D AF NIKKOR or 85mm f/1.4D AF NIKKOR one that has a large aperture so more light can be let through, in aperture priority, with the lowest f-stop you can, you'll increase your chances of capturing a good shot in low light.
If your camera or lens has image stabilisation make sure you use it to help reduce blur.
The longer your exposure, the more light can reach your camera's sensor but how long your exposure is will depend on what you're photographing. For shots where you want movement to be the main focus of your shot, longer shutter speeds will blur the paths any moving objects create while stationary objects remain sharp. Try shooting several test exposures until you find the right aperture / shutter speed that gives you the correct exposure to give the image the 'feel' you're after. If you do struggle to get the desired blur as light levels are too high, however, try fitting a polarising filter to stop more of the light from reaching your sensor. 8 or 10 stop ND filters are handy too but not every photographer has these in their kit.
Really long shutter speeds (above and beyond 30 seconds) can also remove anyone who happens to be walking through your shot. So if you're taking photos in a popular tourist area, for example, but don't want people to feature in your shot, you can 'hide' them from view.
Longer shutter speeds will also blur lights of moving traffic in to long lines of colour. For this, try standing on a bridge above a motor way or at the side of a road in your town / city where plenty of traffic moves through.
For street lamps, you can use a much shorter shutter speed than if you were photographing the reflective light of a lit up building. In town shots where you have both types of light, always opt for the slower shutter speed as if you don't, you'll just have the street lights as your main focus and not the building(s). This can cause flare but this isn't always a bad thing as overexposed street lamps, especially in winter, can look quite good. Just be careful they don't pull too much attention away from the rest of the shot.
Try using Bulb
Most DSLRs will happily create shutter speeds of 30 seconds but if you want something a little longer you'll need to switch to the B (bulb) mode. This setting will keep the shutter open until you take your finger off the shutter button. You'll find a shutter release is handy for this as you'll be able to keep the shutter open without having to keep your finger on the shutter button.
Shoot in RAW
When it comes to post-production, for a little bit more flexibility, shoot in RAW.
White balance – different colour temperatures
Different light sources have different colour temperatures, some are yellow while others have a more green appearance. Auto white balance should be able to deal with most mixed lighting situations but you may find you have to do some post-production work to correct some colours.
If you use a higher ISO you'll be able to use shorter shutter speeds but switching to a higher ISO does increase the chances of noise creeping into your shots. Most cameras now cope quite well with higher ISOs so shooting in ISO800 or slightly higher isn't a problem. For example, Nikon's D600 has an ISO range from 100 to 6400 that's extendable up to 25,600 (equivalent) and down to 50 (equivalent). This enables faster shutter speeds for finely detailed images with minimal noise.
If you do find noise does start to appear, don't dismiss the shots straight away as sometimes, a little bit of noise can add an extra element of interest to an image.