I've spanked my son once, and only once--when he was about 3, he adamantly refused to take the medication he was prescribed--and spat it out, on me, to boot. So I first held him on the couch, and told him he couldn't get up until he took it. An hour passed. Another hour passed. At least he'd stopped screaming--but still refused.
Finally I said, "Look, here's your choice: you can take the medicine, or I'm going to spank you once. Then I'll offer it again. Refuse, and you're getting 2 whacks. See the pattern?"
First time, he refused. Flip, swat, flip. "Wanna take it?" Refusal. Flip, swat, swat, flip. "Wanna take it?" Refusal. Flip, swat, swat, swat, flip. "Wanna take it?"
..."Hey, that didn't taste bad."
"Okay, now go to bed. Daddy's going to go downstairs, pour himself a stiff drink, have a smoke, and beat his head against the wall until he passes out."
Never had to spank him again. Simple fact of the matter is, every kid is different. With my son, he walks all over his mom--unless I'm home. I'm somehow the disciplinarian, even though I'm also the bad influence ("Daddy, can we go shoot bottle rockets?" "Hell yeah, lemme grab a beer and my zippo!")
I don't even raise my voice; it's actually kind of unnerving--I've only ever swatted him that once (his mother used to whack his butt far more often until I convinced her to cut it out)--yet somehow he's terrified of discipline from me. Normally I just cart him to his room for a time-out.
I think it's because I've learned to be consistent about it. It's not like I was gunning for "fear"; most of the time, he'd rather hang out with me even if I'm doing some project that'll leave him bored to tears. But when he's being bad, all it takes is a look, or me to quietly and calmly ask, "What exactly do you think you're doing?"
I guess that's the key--consistency. Don't make idle threats, whether it's timeouts, spankings, or waterboardings. Warn, and then follow through. If you don't, they learn to gamble("Is he serious?")
Corporal punishment is the absolute last resort, and if you're there, you probably f*cked up somewhere along the line. In this case, we let him get away with tantrum-based stall tactics in the past, and it was basically him going for the gold. It's been 3 years, and I haven't had a problem since.