Foot position is crucial and yet because the area in our brain (the sensory cortex) is larger where it is associated with our hands than our feet, and possibly because our hands are more often in their eye-line, its harder to keep our feet in mind. Consequently, not enough pressure is placed on the big toes which anchor us to the holds, or perhaps the pressure is unevenly placed and then we become out of balance.
A simple yet hard to do exercise to correct this involves breathing into your feet. Before you step onto the route, wiggle your toes in your shoes and try to develop a sense of them inside your boot. As you breathe out, imagine the breath travelling down your legs and into your big toes. This can be done when you are sitting at your desk, anywhere really, but regular practice is a must. Then transfer this to when you are on the rock, trying to maintain that sense all your body as a whole and in particular your 'lower corners' (if the hands are your upper corners, your feet are the lower corners). Really focus on what you can feel beneath your toes, are they equally weighted, is your foot horizonal, are you using the big toe as a lever to keep your feet on (particularly if its overhanging).
Give it a go!