Bellows flare occurs when some of the light coming through the lens hits the bellows and not the film. It then reflects onto the film. Light travels in a straight line. Unless you have a lens shade that exactly matches the film (so no light coming through the lens goes elsewhere), which is virtualy impossible, you will get some bellows flare. A lens shade, which you should use all of the time in all kinds of lighting conditions, helps a lot, but the only absolute solution that is guaranteed to work is to use a camera one size larger than the negative size. Bellows flare is the reason that for my 8x20 photographs I use a 12 x 20 view camera with an 8x20 back. If you are using 4x5 film, use a 5x7 camera with a 4x5 back, etc. That is impractical when using an 8x10 so at the very least, use a good lens shade. We use the folding rubber shades available from Harrison and Harrison.
Michael A. Smith