Get two sticks about an inch in diameter and a couple feet in length. Cake your cows, and when the guilty girl is jammed between two other cows snarfing up cake, come in directly behind her. Put one stick between the placenta and her butt, and the other between the placenta and her tail. Squeeze the sticks together and twist, pulling out whatever part of the placenta that comes. You need to work fast before she is on to you. Usually you only get one chance to pull this off, so make the first attempt count. If you can't get the cow done in the cake row, you might have to take her to the corral and do it in a chute, but the same method works well. If you do have her captured, a shot of LA200 can't hurt.
This might sound a bit crude, but a veterinarian told me that it works as well if not better than any other method. It has the added benefit of not introducing germs from the outside going in, as can easily happen if you put a glove on and stick your arm in the cow.
Retained placentas are a prime example of "an ounce of prevention is worth a pound of cure." With proper nutrition and a good mineral program, retained placentas usually aren't a problem.