In my experience, there is some very good advice on this topic. One thing that I do when I see a breech is to put the cow in the chute and basically push the calf back in until I am sure that she is dialated. I belive this keeps the cord from breaking prematurely. Then I hook the chains and jack and pull them as quickly as possible. I agree with the slapping and trying to suck out the airway. I'm not convinced that hanging them upside down or over a gate is the best route. To me, a weak breather does not need all the weight of its intestines shoving against the diaphram and lungs. When pulling a breech, be confident that you can get and quickly. Otherwise I would opt for a C-Section. I am speaking from experience. Back in February we had a cow at my Dad's place that had a huge set of feet coming backwards. Upon examination, I told him that we should do a C-Section. He didn't want to and the end result was a very good 148 lb Paddy o' Malley that lived two days and a cow that prolapsed/tore her vaginal wall. We literally squeezed the crap out of the calf on the way out.
By the way, our vet is 3 for 3 on C-Sections with all cows now producing in the herd, and the calves all sold. If the calf is worth it, go this route in the big cases. The only problem with C-Secting backwards calves is that they have to be rolled inside the uterus, to place them where the uterus can be cut and repaired. Point..... make sure the vet knows what he or she is doing.
There are some backwards calves that I believe that you just can't save, so don't feel bad. If memory serves me right we are 5 for 7 on backwards calves. We have had 2 backwards calves out of 15 for each of the last two years. All of them were big, 148, 133, 105, 107, leading to what was mentioned earlier, in that they don't have room to turn. Of these the 148 and 133 were lost. Each lived less than two days.
Keep your spirits up, and realize this is going to happen to anyone who stays in it long enough. My advice, try to learn from each experience good or bad. It will make you better in the future.