For CR question types that involve a logical flaw in the argument - assumption, strengthen, weaken, and evaluate - it is definitely best to try to think about the answer first. You might not predict exactly what it will look like, but at least define what the logical gap is: "here's the disconnect between the given premises and the conclusion, so the answer will have to address that."
Your process of negating answer choices is also very helpful. You'll move through faster, though, if you have an idea in mind first about what the answer should address.
Here's another tip: the right answer on an assumption question will often be framed in the negative. For example:
... residents will not dump their trash...
... children are not more likely to be sent to the nurse...
... emissions cannot be further reduced by using...
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
