w/ heavy weight not as possible but warming up to connect the mind to the muscle:
1) Contract and squeeze your lats for a bit prior to engaging the pull down bar.
2) imagine your arms and hands as hooks, thus you'll pull w/ your lats and not your biceps.
even try clasping your thumb on top not your typical grip.
3) imagine squeezing and holding a pencil between your scapula at the bottom of the movement.
4) shut your eyes don't think of the stack or in terms of pounds, but feel where and what muscles you are pulling and contracting.
eventually even at heavier poundage you'll still be connected to the contraction of your lats.