Duane came up with a way to teach his daughter to put her shoes on the proper feet:
My daughter (who just turned 4) seems to get her left and right shoes confused more than half the time. Any explanation of left/right or putting L and R on them is far too premature, since she can't spell yet. So what I did was to take a pen and place an X right on the edge of each shoe, near the bottom, up close to the big toe. As low as possible so that it doesn't look silly. The idea, which seems to be working, is that I can now tell her "Make sure the X's touch." So she looks for the X's, lines them up, and then puts her shoes on.
I tried putting the marks on the bottom but they wore off too fast. So where you put them and how big they are is entirely dependent on how much you care about marking up the shoes. Not recommended for the dressy ones, but not bad for trying on the everyday beat-up pair.