Steel. (IMO, of course)
1. To quote Robert Ruark, use enough gun. A 12 ga will throw (in general) a better pattern with heavy (ounces) loads of steel (or lead for that matter). You get the advantage of a shorter shot column for a given weight of shot. The heavier loads have more pellets per load. If you are "on", more pellets will hit the bird delivering more energy. This is a good thing. So to me, if I have to use steel, I'll probably carry my 12 over my 20. I don't have a 16 and don't know what steel loads are available for one. But I'd carry a 16 over a 20 if those were my two choices.
2. On the topic of energy, larger shot delivers more energy at high percentage range. High speed large shot deliver more energy than slow speed large shot. Speed can blow a pattern though, so you have to visit the pattern board. I wouldn't use steel 4 on pheasant period. I'm in the steel 2 camp for pheasant along with Tom Roster.
3. Open up your choke. I think everybody realizes now that steel generally patterns tighter than lead. Use a pattern board and figure out which load gives the pattern you desire in the shotgun you are going to carry.
4. Pick your shots.The trick (key?) to shooting a high percentage is taking shots you know you can make. Give that 50 yard straight away bird a pass; take the 35 yard left to right crosser. Sometimes it's tough to pass on a low probability shot when you are not seeing many birds. Still, IMO, that's better than a cripple that flies off never to be seen again. Also, if you watch and see where a bird you didn't shoot at lands, often times you can go round him up. Particularly if you have a good dog.
5. If you can, hunt with a dog that knows the game and works close. As has been said here, follow the dog and watch the dog. When the dog gets birdy, be ready.
As always, Your Mileage May Vary and I'm sure there are folks that would disagree. These are just the things that work for me when I have to use steel.