as for technically: drill variations of clinches 1000 times, those you are good at, and those you are not good at Physically: weight train your grip strength, forearm , biceps and shoulders. One particular way is to place a gi over the pull-up bar and do pull-ups while holding the gi.
The Under Over clinch? Over hook side should be heavy on your opponents under hook, your elbow should be pointing at the ground. Your under hook side, you should be raising your elbow, so your arm is parallel to the floor, causing your shoulder to drop below his shoulder. You want to force your opponent into a contorted position where his over hook is ineffective, and his under hook is ineffective, you want to be control on both sides. Once you have the position lock low on the body and throw.
