All good points from everyone else regarding use of the pinky (actually last 3 fingers , like holding a bokken), sticky-palm, and the heretofore unmentioned knife hand (tegatana).
However, it kinda depends on which technique you are attempting and whether *gripping for "grim death"* is appropriate or not. Even so, I would say that if you have to "grip" anything, you are probably trying to muscle the technique, rather than using your partners movement.
Sometimes, if your hands are smaller than your partner, you could use 2 hands, one as the bridging support, e.g. kotegaeshi, nikyo, sankyo. (BTW, you can get sankyo from a 2 finger hold, although technically it changes the technique to a yubi tori, but the principle is exactly the same - and works really well on people with much bigger hands).