I use low tack drafting tape. I run a strip across one end of the negative, so that it barely touches: 1/16 to 1/32 of an inch. You can release the vacuum, flip the negative up, inspect the exposure and return it to the vacuum frame for additional exposure without losing registration. The only problem with this method is it messes up your sloppy borders, if you like those. I simply hide them under the overmat.