This is a great question because it speaks to the essence of the reason why the best doubles teams are successful: they’re always moving.
The best players always seem to be moving their feet. Take the Bryan Brothers for example. They are in constant motion, they are never standing still. I believe that active and constant movement puts doubts in your opponents’ heads. If you’re at the net and just standing there, you don’t put any pressure on the person returning serve or hitting a groundstroke. They won’t be worrying whether you’re going to cross or poach.