Whilst browsing through the Fro Knows Photo forums I came across a very handy bit of information. When you are hand-holding your camera then you shouldn’t let the shutter speed go below the focal length of the lens.
For example, if you have a 70-200mm lens and you are shooting at 70mm then your shutter speed shouldn’t fall below 1/70. If you are shooting at 200mm then your shutter speed should be at least 1/200.
Further complication may come from the camera’s crop factor. My D50 is a DX camera. This essentially means that the camera effectively has a focal length 50% larger than normal (read the Wikipedia article about Nikon’s DX format for a detailed explanation) so that a 50mm lens is really the equivalent of a lens with a 75mm focal length. Therefore when shooting with a 50mm lens on my D50, rather than keeping the shutter speed above 1/50 I actually need to keep it above 1/75.
Finally, if you are using a lens which incorporates some form of image stabilisation then you can be a bit more flexible since some of the shake factor will be removed.
You can read the original discussion here.