Apply antibiotic ointment while the kid's asleep
When you can't win the game, change the rules. (In a good way.) From Jeff:
Our 4 year old son was diagnosed with impetigo (also known as infantago, among other things). His doctor prescribed an antibiotic ointment for him, which he did not like AT ALL. He would cry and wipe it off as soon as it was put on his face. Then my wife came up with the idea of putting it on him while he was sleeping; this ended up working like a charm. We put it on him once just after he went to sleep, and again just before he usually wakes up. After a few days, his impetigo cleared up with no more fussing or crying.
I should mention that the impetigo was on his upper lip and nose, both of which are easily accessible while he sleeps. Other body parts may not be so easy to reach, but this is definitely worth trying if you can manage it.
This tip has popped up in other variations -- other Parenthackers have suggested cutting nails, applying lip balm, and other tricky care tasks while kids are asleep.