Don't grip with the knees. Think toes up, not heels down. I find it easiest if my stirrups are adjusted long enough that I need my feet pretty deep in the stirrups to keep the stirrups from coming off - that prevents me from bracing with my legs. I also find it easier if I keep my heels about 3-4 inches in front of my belt buckle - I hurt my lower right back 5 years ago and it is still too stiff to absorb much of the horse's motion, so I let my waist absorb the motion.
Most people riding western prefer to put their weight on their 'pockets'. I honestly prefer to lean forward some and carry my weight on my thighs. I'm used to English/Australian saddles, and having my weight on my pockets feels weird to me. Besides, the horse's back pivots around the withers at a canter - the loin has far more motion than just behind the withers.
No, the picture below is NOT me...it dates back to the early 1900s: