When film is laid directly on a flatbed glass bed, this often causes an optical interference called Newton's Rings. These are colored rings not unlike the colored patterns seen on oil films on water. These rings are typically small in area, encircling a point of direct contact of glass and film. The patterns are caused where the separation between film and glass is microscopic, thickness of the air gap in the order of a wave-length of light (causing interference patterns). The slightest separation, or no glass at all, is the answer. A thin spacer to hold the film surface slightly away from the glass will prevent Newton's Rings (but not so far as to affect focus). A thin cardboard or plastic frame, similar to a 35 mm slide mount, with a cutout for the film size, or maybe two L shapes, can provide an opaque border, about 2 inches all around the film. This border also blocks stray light coming around the film edges, shielding the lens and CCD from stray light and flare, which provides better image contrast.
Film has the correct left to right orientation viewed from the back shiny side, so the film goes emulsion side up on a flatbed. Scanning with the dull film emulsion side down (against the glass) may help a little to minimize Newton's Rings. The image will be reversed, but you can easily flip it later.
Large photo supply stores have "mounting oil", much like mineral oil, used on drum scanners to fill the microscopic gaps between film and glass. This eliminates Newton's Rings, and it fills and hides scratches too. The cleanup problems are a serious issue for casual scanning. Placing film in an old phone book afterward for awhile works pretty well as a blotter. Or if available, Kami fluid is not oil, it is more like alcohol, and it works well for this too, with only minor cleanup problems. These methods are surely a bit much for casual scanning.
Microtek makes flatbed models with a film drawer using glassless film carriers which eliminates Newton's Rings (the 8x10 inch tray insert does have a glass bed, and odd film sizes lay on that, but 35 mm, 6x9 cm, and 4x5 inch film sizes instead use sized glassless film holders). The film drawer will prevent scanning oversize film however, it must fit in the tray, no overhang. Film scanners use glassless carriers too of course. Darkroom enlargers and slide projectors have used only glassless film carriers for many years, for this reason.