One thing that I've found useful for removing adhesive is ... adhesive! Take some tape and press it down on the recalcitrant adhesive, and then lift up. You may need to do this a few times. I've found it to be quite effective.
Another thing you can do is not use tape. I've found that a water-soluble temporary glue stick is just fine for temporarily fixing a sheet of film in place, and of course it comes right off.
Another thing I saw is one photographer modified one of his 8x10 holders to have a suction tube on the back. When he wants to make his exposure, he puts the tube in his mouth and inhales slightly, and the film stays flat for the exposure.
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