This is an interesting topic I actually struggle with. I am by no means opposed to spanking as I indeed find it to be a very effective means of discipline. In fact, just recently I've been encouraged to take up "the switch" as it is extremely effective. The struggle arises from many things. The first is the ability to discern what is worthy of discipline. Pretty much any disobedience is worthy of disciplining but deciding where to draw that line takes wisdom (something I often lack). Trying to judge that line with another adult is also very difficult as what may be inappropriate for one may not be for another.
Another area of struggle is the idea that using violence is the best means for behavior correction. Indeed it is effective at any degree in life but one has to question is it the best way to generate that sought after behavior change or obedience? Did its use in earlier eras explain some of history's brutality? The fear of what others may think or worse that someone might actually report you for abuse is always at the back of my mind. "Time outs" do prove somewhat effective and are the most common way I discipline. But I confess, sometimes it feels fruitless after 20 minutes of trying to force them to stay in one spot or they repeat their disobediance moments after being released from time out.
I find it ironic though that if we were to lock up our children like we do adults who disobey rules we would also be punished. It takes surprising strength to withhold and restrain my anger when I feel an act is worthy of a spanking but at other times it takes tremendous courage and wisdom to strike a child in faith that it is for their own good. In the end, this aspect of life requires faith and God's grace to raise a child correctly. It requires me to overcome my flesh that desires me to be lazy and "let it slide" and to balance it with controlling my temper and not harming them. Truthfully, the method of discipline is as less important as compared to its consistent execution and its result.