Caporegime
Delvis, really can't see why the bar should slip from your hands in squats! Can understand chalking your back as ice says, but only under HEAVY loads. Haven't felt the need to do it yet and I'm on ~140 working sets.
It doesn't slip FROM my hands, I grip the bar and pull it down so sometimes me hands slip inwards. Doesn't help I use the thumb over the bar grip.