Common Sense's page

Intimidate is not the only skill that can raised really high. Stealth is worse, with some rather easy to get bonuses in excess of 20. Lots of skills can, and often become so high that any skill check is trivial.

If you can't picture it being done, and said action is not found in the rules, then it wasn't intended to be done.

Can you picture using a meteor hammer against a prone target?
"This weapon consists of one or two spherical weights attached by a 10-foot chain. You whirl the weights and wrap them around an opponent's body. If you succeed at a trip attempt with a meteor hammer, you can drag your opponent 5 feet closer to you rather than knocking her prone." How exactly do you plan to wrap your weapon around a prone target?

Can you picture getting an AoO form trying to trip someone who is already prone? How would you even do that? Isn't it the same as just hitting them?

If you could trip someone who was prone, I guess you could also:

Sunder an item on a person who has no items on them (Yes, that person is naked).
Steal an item on a person who has no items on them.
Disarm a person who doesn't have anything in their hands.
Etc...

In order to be tripped, you need to be able to stumble or fall (stumble meaning to almost fall).

Google defines the terms as:
Trip: "catch one's foot on something and stumble or fall."
Stumble: "trip or momentarily lose one's balance; almost fall."
Fall: "move downward, typically rapidly and freely without control, from a higher to a lower level."

It is not common sense, it goes against the English language, and it's not described in the rules. What more is needed?

I say that you can attempt to trip a prone target.

But I also say that you will fail.

"Trip: When a prone character stands up and provokes an attack of opportunity, can I use that attack to trip the character again?

No. The attack of opportunity is triggered before the action that triggered it is resolved. In this case, the target is still prone when the attack of opportunity occurs (and you get the normal bonuses when making such an attack). Since the trip combat maneuver does not prevent the target's action, the target then stands up."

So, yes, you can try it. But it doesn't do anything, you automatically fail.