i tend to think of body mechanics too.. ex.. when I do say incline dumbell presses, I push from my "elbows" and it is just a pivot joint at the elbow and try to think of my triceps just keeping the weight (elbows) purpendicular to the floor (holding it in place not pressing so much) and let the weight press up from elbows, (chest muscles)
in squats, i think of the lower back like a tricep. let the weight go down using your legs and any bending over by back is just to maintain balance. using the 2 x 4 helps alot in keeping the weight over heels, (CENTER OF GRAVITY), I bend over TOO much when I dont use one,
(I hope that didnt sound retarded)... but I do the same with lat pulldowns too, pull down from elbows and I have been able to almost keep biceps limp for the first few reps cuz it is all from elbows or machine presses for shoulders, I dont think of straightening arms, I think of the arc movement elbow makes and I use less tris. (sorry if I digressed)