I use the Epson Perfection series scanners to convert slides. The scanner does an excellent job. It provides a 35mm slide holder and, most importantly, can shine light through the slide (most scanners can only bounce light off the image being scanned).

There are definitely a few options, ranging from inexpensive (but lower quality) to high-quality (but expensive). For all of them, the trick involves backlighting. If you’re not willing to invest the few hundred dollars it would cost to get a proper 35 mm slide scanner but you have access to a regular scanner, you can get a slide attachment that will provide the backlighting as part of a slide-holder — the problem is that you have to scan the slides at a fairly high resolution, and if your scanner isn’t so good your results can be iffy.

If you have a good digital camera and a fairly stable tripod, the hack of choice appears to be finding a slide lightbox (or building your own) and shooting the slides with your digital camera. There are some details on the process here, put together by model railroaders who had the same problem: http://www.modelerschoice.com/Articles/slideconvert.htm