Fence foundations are VERY different from deck foundations. Deck foundations are almost always made with concrete, at least at some point in the process. Fence foundations are completely opposite of that. Using concrete for fence posts would be a complete waste of time and resources.
The number one rule of thumb when planting fence posts into the ground is this: never bury a post in concrete. Not only will burying wooden posts in concrete slow down the process and cost more, but it will actually speed up the rotting process. Water gets into the hole in the concrete, and does not escape.
So now you know what NOT to do, lets talk about what you should do when dropping fence posts. First, dig the fence post hole as close in diameter as you can to the width of the post. You do not want a lot of space surrounding the posts. Post depth is vital to the strength of the fence itself. You will want to dig a minimum of 3 feet deep for a standard 5 foot fence. So you can do the math, you will need 8 foot posts for a standard fence foundation.
At the bottom of your fence foundation, you can place some gravel at the base of the hole so the post is not resting on soil alone. Back fill the hole with soil and gravel. Tamp the soil down as you work your way up the post until you reach ground level.