Here's a video about this method.
Step 1: Traditional Method
I stopped using it a while ago, I care about my customers and myself.
Step 2: Reconcreting?!?
when you dig up the lump of concrete you generally end up with a much bigger hole and you might have to use a lot more concrete as shown on the picture to fix the new fence post. See for instance the havoc created at minute 0:50 on youtube video http://www.youtube.com/watch?v=9A8i6mouohw
More work, more material, more costs. All this can be avoided.
Step 3: Keep a Gap!
this is a way to re-use the concrete foundation
Step 4: Apply the Spacers
The spacers will keep a uniform gap between the post and the bag
Step 5: Insert the Post Socket Bag
the socket bag is specifically designed for the purpose of fitting a 100x100mm (4"x4") section fence post. The bag is fairly robust and 900mm (3ft) long to cover any eventuality and ground conditions but can be easily cut to a shorter lenghth
Step 6: Fill the Gap With Dry Sand
fill the gap with dry sand and pat while doing it to create a grip between the socket and the post
Step 7: Insert Some Foam on the Top
the material used is off cuts of insulation pipes, this is to separate the silicon from the dry sand
Step 8: Silicon It
seal the top with silicon to keep the water out.
Step 9: Concrete It
insert the post into the ground and concrete it in.
Step 10: Suck It Out
to extract the post, cut the silicon seal and suck the sand out with a vacuum cleaner
Step 11: Replace
now you have a hollow foundation and can replace the post re-using the dry sand.
Step 12: Concrete Out?
should you need to remove the concrete foundation completely due to ground movement or change of plans, it is still far easier to do so as the hollow fondation can be broken into pieces with the aid of a metal spike like I did in this trial. And the hole on the ground keeps more or less the original shape without getting bigger as it happens when removing a solid block of concrete.