Probably the stupid WIND. You could go barefoot like the guys on the saltwater skiffs do. Or you could use a laundry basket on the deck....Or, when a perfect double-haul really matters, you can leap 3 feet in the air, tucking your heels into your butt cheeks, as you make your final hail-Mary throw. ð
Seriously though, what works best for ME is to just make a quick glance down to inspect the mess on the deck before each cast. And just concentrate on not moving your feet from that point until all the line is out of the boat. The style of shoes you wear matters too. The absolute worst are any sandals, flip-flops or anything with velcro flaps instead of laces.