"Weaken" does not mean "prove false".

A correct choice simply needs to show that the author's conclusion might be a little shaky. Even if a choice casts just a tiny bit of doubt on the argument, then it is sufficient. Also, only one choice will actually weaken the argument. If you ever find yourself with two choices that both seem to weaken the argument, do not try to weigh which one weakens more. Instead, try to determine which one does nothing to the argument, and is thus incorrect. See (E) on LSAT PrepTest 50, Section 4, Question 5.

Do not attribute a conclusion to the author that s/he does not make.

Avoid the temptation of guessing at what the author's conclusion might be, and then trying to weaken that conclusion rather than the author's actual conclusion. Consider LSAT PrepTest 47, Section 1, Question 2. The author concludes that efforts have been wasted, but does not take an extra step and state that such efforts should be stopped. If you do not recognize this and guess that the author wants the efforts stopped, then the incorrect choices might seem attractive.

 

EXCEPT Questions

The correct choice does not necessarily need to strengthen the argument; it could simply do nothing to the argument. As an example, see LSAT PrepTest 48, Section 4, Question 23.