Well, nothing I say is going to keep it from being one of the hardest question types on the test.
1. Are you comfortable negating?
Because Necessary Assumption is really asking us, "which answer, if negated, most weakens?"
2. Do you find Premises and Conclusions easily by using available keywords?
PREMISE indicators (F.A.B.S.) = for, after all, because, since
(these are attached to premises, but this premise comes immediately before or after a conclusion)
CONCLUSION indicators = so, thus, therefore, hence, this shows that, because of this, clearly then
3. Can you tell when they're testing Idea Math (bridge ideas) vs. Courtroom Debate (defender ideas)?
Some correct answers are testing Idea Math, our ability to compute the intended logic of the author's argument.
Other correct answers are dealing with Courtroom Debate, they're ruling out a potential objection someone might make to the argument.
Idea Math answers sound like connective tissue or missing ideas. The most common kind is a Bridge idea that connects a Premise concept to a Conclusion concept. But there might just be a missing fact that's integral to the logic.
f.e. Danny isn't going to be allowed to come to Jenny's party. After all, Danny is only allowed to come if his brother Paul is allowed to come.
(assumption: Paul is not allowed to come)
When we negate an Idea Math answer, it will weaken the argument (just like any Necessary Assumption). But often we can judge the correctness of these answers simply by judging how they fit into the author's thinking. Many trap answers seem to offer Idea Math ideas in conditional logic form. Here, you have to be really vigilant about making sure they're not presenting the author's thinking as an illegal reversal or negation of what it should be.
Courtroom Debate ideas usually feature the word "not" or sound like they're ruling out an idea. They will bring up concepts that have not been talked about in the argument core. These are the answers that are really hard to predict, and we usually don't realize what they're saying or what they're worth until we negate them.
EXAMPLE:
Joe applied to Notre Dame. Thus, Joe must want to go to a good school.
IDEA MATH assumption:
Notre Dame is a good school.
DEFENDER assumptions:
Joe's parents didn't force him to apply.
Someone didn't hold him at gunpoint and force him to apply.
Joe isn't going to Notre Dame simply because his girlfriend is going there.
(and on and on)
Hope this helps.