The hardest thing about these types of questions is that you must always, always only use what is given to you in the passage. Sometimes you may have a personal feeling about the passage you are reading and that can influence your answer choices. The inference and assumptions questions are often structured to exploit that fact. In addition, these questions are usually structured that you have to use two specific facts or opinions listed in the passage to arrive at a logical answer.
The easiest way to get a handle of these types of questions is to try and take the author's perspective and ask, "What is the most logical progression given what the author has written/knows?"
Put yourself in their shoes and try to make their opinions your own for just a minute, and then look through the passage for clues as to what the most logical answer is.