Never even allow a student to ask a question during a test or quiz. This is perhaps the most important piece of teaching advice I can give.
I am utterly convinced that your school could raise your SAT scores by 20 points across the board, and your AP math/science scores by a third of a grade, merely if your math and science departments never allowed questions during tests.
It is a dirty little secret that no one ever discusses... so many teachers talk their students through difficult problems. No wonder those students struggle when they're faced with standardized tests, when their friendly lifeline is taken away.
I hear people argue with me, saying that they answer questions on tests because they want to help the students succeed. Well, so do I -- and I take offense to the ridiculous connection that refusing to answer test questions equates to not caring about students. I want my students to succeed over the time frame of their physics course. That doesn't mean they must ace every individual test or quiz. It is crucial that we allow our students to make mistakes, and then to learn from those mistakes. I judge my success by how well students perform at year's end, not by whether one student got one question right on one test.
Here is the critical post explaining my approach, including some help with the issue that I know many of you already brought up, that "I could never get away with this at my school." :-)