Keep Slides clean. Use lint free cotton gloves when handling negatives and a can of compressed air, both obtainable from most camera shops, to prevent dust getting into the scanner.
It is generally accepted that you should try to achieve the best possible scan before TWAIN transfers the image to the host application that launched the scanner software (by using the import command). This is because most scanners have a higher number of Input bits (typically 30) to output bits (typically 24, or 8 bits per RGB channel). Hence, if any interpolation or colour adjustment needs to be done when scanning it is better to do it using the maximum amount of information available.
If you have an external scanner, such as the Minolta Scan Dual, and forget to switch it on before going into Windows, you don't have to reboot the computer in order to make it work (actually for the SCSI card to recognize the device). Simply Right Click on My Computer icon on the Desktop, click on the Device Manger tab and then click on the Refresh button at the bottom of the dialog box.
If the scans don't look right in Photoshop then ensure that you have set the scanner options properly, in particular the histograms. If you haven't printed and read the manual on the scanner CD and fully understand how to use histograms, do it now. Using histograms can help to overcome a badly adjusted monitor setup, the graphs showing what colours the computer system knows about. Trust it more than your eyes.
Basically if the scanner fails to work check, the mains lead is connected (I failed this one a few days ago at a computer club meeting), the mains switch is on, and the system is seeing the SCSI card correctly. One other thing with Nikon film scanners, is to ensure there is no dust inside - spray with compressed air into the slot and it can magically burst into life.