Digging Post Holes
One way to dig your post holes is with a clamshell digger. This is pretty hard work, and if you've got a lot of holes or hard ground you should rent a power auger.
It usually take two people to run it. Even with the power auger, digging post holes is tough work.
In most areas, you want your posts below the frost line, so when the ground heaves from frost, the post will stay in place.
Our building codes specify that posts be dug down 42 inches. We actually only need the corner posts, or posts around a gate that deep. For our middle posts, or "line posts", we dug down 36 inches.
Obviously, in warmer areas, the frost line isn't this deep. A good rule-of-thumb is that you should bury about a half of the corner posts, and about a third of the line posts underground.
Redwood, cedar or treated posts will resist rot and decay, but it's still a good idea to put a five or six inch layer of gravel at the bottom of each hole.
The gravel will allow any water that collects at the bottom of the hole to drain away.
Set the end posts first. It's a good idea to use concrete to set these.
Line up the post edge along your layout string. You should also use a 4-foot level to make sure that the post is plumb in both directions.
When you get to the top, fill the hole up a little higher than the ground, and slope the top of the concrete away from the pole a little. This will keep water from running down between the post and the concrete.
Brace the post in place while the concrete cures.
The line posts require a lot less support. In most cases it's ok to set them with dirt.
Use a spacer to get a consistent distance between the posts. When the post is in position, shovel dirt in around it.
Once you've got three or four inches of dirt in, pack the dirt down, using the end of a 1x2, then keep filling it up.
When all the posts are set hard, trim the tops to the proper height.