What Bryan just mentioned there sounds interesting, although I have as yet to receive a painless nikyo from anyone... yonkyo maybe.
I've been taught before to try wrapping ukes forearm.
lets say you're ai hanmi. left leg and hands forward. and uke grabs your hand with his left as well.
nage right hand clasps uke's left hand not strongly but surely. nage's left hand climbs up on uke's upper wrist like a dolphin (little finger) jumping out of the sea and back again on the other side of the wrist. (fingers point down now.)
at this point in time, your center takes ukes center down. the control of ukes center comes from your hands on his hands. his elbow should be bent almost 90 degrees. note, that you shouldn't try to twist his hand down, but instead use your center/body to cut his body down. Thats the whole reason on the 'dolphin' thing. If you focus away from trying to manipulate his wrists, and instead just thinking about doing whats right with yours, you could accomplish this technique.