One of my darkroom tricks, is to use removeable magic tape to hold the neg from even the slightest movement in the neg holder, when the holder with neg, is being inserted into the enlarger.
If I have a curling neg I carefully tape the rebate of the neg along one side. Then, very carefully, I apply tape to the opposite side of the neg, after this is done, I carefully pull the neg flat and press the neg and tape onto the neg carrier.
Be careful that you don't distort the neg by pulling diagonally, thereby causing the neg to be stretched, making your undulations worse.
Ensure that you use removeable magic tape.
Normal magic tape sticks like crazy, and is a devil of a thing to remove from your negatives.
Removeable magic tape, is the same adhesive as on the Post It Notes.
As a bonus, removeable magic tape, is quite brilliant at removing dust particles from a neg.
MIck.