Post expert here
In my orchard we probably have installed in excess of 20,000 treated wood trellis and deer fence posts up to 6" in diameter several different ways depending on how many we had to do.
In my area the rental yards have hydraulic pounders like this
http://www.wheatheart.com/hitter.html that they rent with or without a tractor. Sink the post two feet in one to three smacks, no hole digging or tamping. Posts are much stronger than a dug hole. Have done over five hundred a day with one of these. The posts must be pointed, rocks don't matter, they get pushed out of the way unless you are right on top of a big one. Best with two people, one to drive posts, the other the tractor.
BTW, the hydraulics don't actually drive the post, they just lift the 100 lb pounder head up, four humongous springs pull the head back down at what seems like 400 mph.
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The method we use for just a couple hundred is to use water to make the hole. Have a tee shaped handle with valve connected to 100 psi water source, we use a sprayer. The head of the handle is made with a 4'' long piece of pipe the same diameter as the post with a pointed end with a 1/4'' hole for the water at the tip. Takes two people about 30 seconds to set a post. One guy blasts the hole and the instant the hole is deep enough the other person drops the post in and levels it quickly. The soil suspended in the water instantly settles and sets the post like it's in concrete, you have to work fast before the soil settles. Half a shovel of dirt and the post is done. One advantage of this is that gravel settles at the bottom of the hole, making an excellent base.
When we have just a few posts to do and dig the hole by hand or with a tractor auger we just kick the soil in about 3/4 of the way, pour in water then fill the rest of the way with soil and tamp. All we use for a tamper is a 3/4'' pipe with an inch square piece of plate welded to it, don't need the weight when you use water. With a slight bend in the pipe your hand doesn't skin up against the post. Used to use a shovel handle end to tamp but it wears them out fast. When I suggested my 30+ lb bar for tamping to my strong,young Mexican workers they just shook their heads no.
Another post trick. Use a couple sheets of toilet paper to mark where the post goes. One roll will get you a couple miles. A lot easier than fooling with flags or lime. You have to make the holes rather soon though or mice will haul the TP away for their nests.
We also use a level with two vials mounted to a piece of angle iron, checks the level of both sides at the same time, attaches to the post with a Velcro band. Many hardware stores have them.