http://www.ehow.com/video_4419369_orchid-root-care-maintenance-part.html - video on repotting an orchid by Gayle Zubek
The key to growing beautiful, healthy and thriving orchids requires keeping the root system fresh and clean. Trimming the roots and cleaning the debris around the roots keeps the orchids healthy. About every six months set some time aside to trim and clean the roots of your orchids by following steps.
1. Clean all the old potting mix from the orchid roots. Lay the roots out on a newspaper or some other type of covering that you throw away when done
2. Take a look at the orchid's roots. Notice the rotted or dead roots. Differentiate between the green colored, healthy roots and the yellow stringy dry roots or the hollow, dry mushy roots. Determine where you need to cut the old roots off the root bulb
3. Cut the roots from the healthy section of the root system. Make sure to cut a little further from the point where the rotted area stops to ensure removal of all decay
4. Consider what you want to achieve by cutting out the rotting roots. Orchid's root system needs air flowing through. The less gunk between the roots better the plant thrives and creates new blooms
5. Repot the orchid as soon as you finish cutting all the old, rotted roots off. Use fresh potting mix instead of the old mix.