One thing that helps is if you really use your legs while paddling--so that you're always pushing first one foot then the other against the brace, then releasing, then pushing, releasing, etc. This leg action not only adds a lot of power to your paddle stroke, it also helps promote blood circulation in your legs and especially your feet.
There are definitely times, though, where I will take my legs off the foot braces and stretch them out fully, wiggle my toes and feet--which quickly restores them. Not advisable in rough conditions, though!