pits always work due to the lack of mobility and basic hand-eye coordination the common zombie has. but you may say to yourself hey a hole in the ground isn't enough so here's the solution... ideas... you can use whatever you want:
or even the same earth you took away from the hole it doesn't matter
A simple trap. Take a normal mouse trap, some string and a 12gauge shell. Drill a hole in the trap and slide the shell into the hole string it up and bam! You have a ghetto AP trap. I left some of the details out but you can figure out the rest I'm sure. It may or may may not kill the zombie but it would slow him down and offer you a early warning.
You have to be careful when you do it of course. Once you slide the shell in (it has to be a very tight fit mind you.) you may need to either bend the tip of the trap or attach some thing to the trap so that it will spark the primer. Then all you need to do is attach the string to the trigger arm it very very carefully and thier you go. I would not recomend going for the head with trap since Z's are not of uniform height, maybe a top down style would work come to think about it. If you have trouble getting it to go of try a rat trap in place of the mouse trap.
Fit a small length of metal pipe to it, maybe 8-10 inches or so. Place the shell in that. This will help focus the blast and keep the shell from exploding. It will still have a decent spread (think sawed-off shot gun) and increase the lethality greatly. Place about 5'4" off the ground to maximize a headshot.
A side note to this. These do not descriminate. Other survivors or your friends or even yourself run the risk of setting this off. Always disable, if possible, when moving to a new place.