i got a private message regarding this thread some time ago.

regarding "attacking the premises":
one thing that you WILL NOT see in the passages on this test -- even though it happens in the real world all the time -- is *stated facts* that later turn out not to be true.

here's an example argument:
[i[Jim eats more food per day this year than he did last year. therefore, Jim will gain weight this year.[/i]
IN THE REAL WORLD, one very ordinary way to weaken this argument is to say that the original premise is actually a lie -- in other words, to state that jim actually doesn't eat more food per day, and that the first statement is thus erroneous. if that's the case, then the argument is very much weakened, because the entire premise upon which it is based has been shown to be a lie!
HOWEVER ...
ON THE GMAT, no known official problem has ever contained stated facts that are then contradicted (either later in the passage or in the answer choices). therefore, you can trust that weakeners will NOT attack established facts.

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ON THE OTHER HAND
you want to be careful with the word "premise" here, because that is still a vague word -- "premise" could refer either to a stated FACT or to a CLAIM that is drawn in the middle of the argument.

for instance, here's another argument:
Your boyfriend has stayed late at work for the last five days. Therefore, he's probably cheating on you. Thus, you should leave him.
in this argument, the FIRST premise (Your boyfriend has stayed late at work for the last five days) is stated as a fact, so it's a fact; we aren't going to contradict this one.
on the other hand, the second premise (he's probably cheating on you) is NOT a fact; this is an inference that the author draws from the first fact, so, even though it's a premise of the argument, it may well be false.
for instance, an answer choice saying "his company's fiscal year ends tomorrow, and employees typically work long hours at the close of fiscal year" would weaken this argument -- because it contradicts the claim in the 2nd premise. i.e., he really is staying late at work so that he can work, not so that he can cheat.

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short summary:
weakeners won't contradict premises that are facts.
weakeners *may* contradict premises that are claims.

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by the way, memorizing these things from books like powerscore will *not* help you; it will actually hurt you. see, everything i said above is just common sense -- i didn't have to "study" to figure these things out (this is also why i am using real-world examples that are not academic arguments).

if you STOP TRYING TO MEMORIZE RULES from books like powerscore, and instead just step back and consider these things through the window of common sense, you will probably discover that you, too, already know all of these things -- and that the "rules" you're trying to memorize are much, much less accurate than your own common sense.