I taped the negs to each other with small (~1/4 of a thumb nail) weakened gaffers tape. I weakened it by placing a fresh tape on my cotton shirt, which was still sticky enough to hold the negatives in place. The tape was mainly beyond the borders of the paper, so it wouldn't show up on the contact sheet as a white spot (except for one sheet, during at the end of the run and got sloppy). Then I used a 20x24 sheet of 4-5 mm-thick glass to weigh down the film strips.
Originally Posted by Dr Croubie
I thought about using clear tape, but decided it might leave residue. Then I tried it with the cotton shirt method and it wasn't sticky enough.