I beg the differ gentlemen.
not lower on your back I say higher!
focusing on the heels is for deads and back squats. here u wont need it.
If u want to squat staright up and down as an olympic lifter does, the higher the bar goes, the more every inch of forward bend counts in moving your centre of gravity and thus allowing a lesser forward bend.
When I changed the position of the bar to my shoulders, I added instant pounds also and was able to squat deeper easily!
The trick is (although it demands forearm and shoulder flexibilty - if u ever heard it mentioned with squat and couldny imagine why now you will know)
go under the bar, place your palms pretty wide, and raise your elbows till yourms are paralel to the floor. this will make the bar “sit” on your shoulders. this is gonna hurt at the begining, especialy since u can suddenly use more weight… suck it sissy! or u could be smart and put a towel. dont let the elbows drop during the rep.
U will probably find that your stance will vary a little bit (u may have to flare your knees more) but u can go deeper easily, and keep that lumbar arch all the way!
hope this helps.