This is it: everything you've ever wanted to know about yoko-sankaku. It's here.

Key points:
a) Secure the keylock before trying to secure the sankaku
b) Once keylock is secured, readjust the legs to get a good sankaku
c) Make sure uke's head is resting on your right thigh (if his shoulder blades are on your thigh, the positioning is no good)
d) If the strangle doesn't work, go for the armlock and if that doesn't work, go for the hold-down.