I'm no expert, but here's a few tricks that have really helped me.
Most passages follow a pattern
Premise + Assumption = Conclusion
Premises are FACTS that you don't mess with and are STATED in the passage. Don't question premises.
Assumptions are UNSTATED and help the author reach a conclusion.
So take this example.
The State of California is decreasing the minimum drinking age to 15. As a result, Californians should expect to see more drunk driving deaths.
The premise is "The State of California is decreasing the minimum drinking age to 15. " Don't mess with it. Just take it for what it is.
The conclusion is "As a result, Californians should expect to see more drunk driving deaths."
The assumption is how the author arrives to thinking this. The author ASSUMES a few things like: parents will allow their children to drink and drive, that more 15-21 year olds will actually want to drink, etc.
So if you strengthen the argument you usually with have something that solidifies an assumption. In this case, one way to strengthen the argument would be if you found out that 90% of 15 to 21 years olds have their own car to drive for social purposes in the State of California.
You would not strengthen the arugment with something like: "The State of California as passed the law." That affects the premise.
Strengthen/weakening of arguments generally attack the assumption.
REMEMBER that assumptions are the UNSTATED way that the premise connects to the conclusion.
Practice by identifying the premise and conclusion after reading.
