Scanning photo slides and scanning film negatives are pretty similar processes. You need to clean up the scanner bed and film slides with a dry cloth. Get proper backlighting using a bright white glossy paper that can act as a reflector. Put the slide on the flatbed scan and put the backlighter box over it. Adjust the scanning software to the highest resolution. Set a high DPI and increase the brightness. You can set the brightness and contrast using a photo editing tool too or increase the image size to serve your purpose well.
Saratoga Slide Scanning : The Most Professional Way to Scan Slides
Saratoga Slide Scanning : The Most Professional Way to Scan Slides
Source: http://www.techhive.com/article/211222/scan_slides.html