I have a 6 month old son, and my wife and I are deciding how to discipline, and how to be consistent about it, which we believe is extremely important. A lot of it will have to depend on his personality as it develops.
I have, however been around horses for all of my life, and I grew up in the new "Horse Whisperer" type of mentality. For those who don't know, it's against the idea of "Breaking" a horse's spirit, but instead tries to gain the trust and loyalty of the horse, so that it wants to please you. Instead of a master/slave type of relationship, you want a rider/friend type of relationship.
Something that I learned from that regarding physical force as punishment is that horses can take a lot before they submit, and if you rely on that with a strong-willed horse, then you have to always increase the pain to keep the horse obedient. You have to get harsher bits, meaner spurs, and more and more contraptions to keep the horse in check. This is a bad tack to take. (Whoops, no pun intended).
Instead horses hate to be left out of the herd, that's a more effective punishment for them -- to feed them last, or to cut short a training time and put them in a stall while you are paying attention to a different horse, etc. But if you just ignore them, they never learn what they did wrong.
So, with horses, (and my current plan is to try this with my son, since we're ultimately herd animals ourselves), when they do something wrong, you have to let them know right away that you are displeased. Often this is a quick pop on the neck, or making noise, or sometimes even just waving your hand at their face, but it never is painful or degrading, it's just an instant message that you are unhappy.
Anyway, I have rarely spanked my son in 12 years, maybe once or twice max. However, a pop on butt is not out the question when they are young. When a child is very young they have a very short attention span. At that age, a quick pop (not a spanking) on the hind end is an extremely effective way to get their immediate undivided attention!
Then, after that is when you administer whatever actual punishment is appropriate (for horses, a "time out" works very well). You HAVE to be firm, or the horse learns that you can be manipulated. After the punishment period, you then HAVE to take the time and re-unite with the horse, to "let them back into your herd".
In this way, the horse learns that 1.) What behaviors are appropriate or not, 2.) That you can't be manipulated, and 3.) Even if you are upset, the horse still has your unconditional acceptance in your life not based on performance.
It's a time consuming process, and it's a pain when they test you right as you are about to leave for work, and you have to go through the whole process, but horses are too big and dangerous to just let that stuff go.
If it sounds terrible suggesting that I treat childred like animals, remember that the guy who is the current guru on this subject, John Lyons, also takes in foster kids, usually middle-or-high-school boys, and uses variations of this idea with them, with some very good results.
As kids get older, there are different ways to give them that "instant feedback", which may or may not be spanking -- I think it depends on the kid's temperment. But when they're little, they just don't understand reasoning or consequences yet.