Take dozens of pictures of different feet. Shrink 'em and print them out in black and white. Draw from the picture (or trace if that doesn't work out). Then just keep drawing them. Over and over and over and over. Don't forget about ankles. A little triangle where the ankle belongs does WONDERS. Also, unless someone's in running blocks or something, the line from the calf to the tendons in the back of the ankle to the base of the foot is always curved.
Stare at feet, and you'll draw better feet. I promise.
