We didn't babyproof much with our now-3-year-old, because she just wasn't interested in finding out what was under the sink or in the toilet or in the kitchen drawers. But our 18-month-old -- we've nicknamed him "The Menace." He is into EVERYTHING.
With all of our girls, when they tried to do something they shouldn't, we'd say "No, no" and they'd stop. With both our boys, though, we say "No, no" and they try to do whatever they were supposed to stop doing three or four more times before we can get over to them and physically remove them from the area. Our youngest takes things a step further... remove him from the area, and he'll watch you and then go back to what he wasn't supposed to be doing and do it again -- WHILE STARING AT YOU AND GRINNING.
So... I've got makeshift baby locks on the cabinets, we keep the bathroom doors closed, and any toy with small pieces must be played with upstairs in a child's bedroom and not in the family room (so he won't eat the pieces) and must be cleaned up immediately when they're done playing.
But you're right, it's impossible to catch everything.... Yesterday, I changed a diaper that was full of chewed crayon that I didn't even know he'd ingested. Gross.