Some practical points:
- Spend the time and re-read the exam text "adversarially", trying to think up any possible way to misinterpret them, and correct for it. On the other hand, better to leave some option for mis-interpretation than spending half a page on spelling out the interpretation.
- Have another teacher/TA in the course sit down and solve the exam in its entirety, both to catch potential errors/unclear points and to time it and assess difficulty.
- Do answer trivial questions (e.g. confirmations) briefly, but kindly and in a supportive rather than derogatory tone. Example: Q: "Am I really supposed to do XYZ?" A: "Yes, those are the instructions." Don't try lecturing people about asking such questions; don't repeat the demand not ask such questions. Instead, try to smile (if you can manage doing that in a non-creepy smileway), expressing the pride of having brought your student from relative ignorance to the brink of being proven to have mastered the subject matter. I always felt proud of my students taking their exam!
- Remember that students' questions while taking the exam may uncover an unforeseen problem with it. This has happened to me quite a few times - both as a student as a TA.
- When faced with a "How do I solve this?" question, and declining to answer - have a look at the student's face. If they appear super-stressed, shaking, desperate - consider suggesting they try moving on to the next question/problem and go back to this one later; or that they go have a drink of water and catch their breath. Consider asking "Are you all right?" if you're actually concerned about them. Some people occasionally have 'blackouts', or otherwise become very anxious, and this sometimes helps. It actually 'discourages' useless questions in my experience, because even noticing that a stressed-out student is being attended to with a kind demeanor reduces stress somewhat. That might sound irrational, childish, overly soft, immature to you - but remember you're testing the student's command of math, not the quality of their character or their maturity. Plus - perhaps, for you, an excellent academic who made it to professorship the exams were not that hard and stressful as for other students - don't judge them until you've walked a mile in their shoes.
- If you get a lot of questions overall, or about some point in particular - when the exam ends, either stay there for 10 more minutes and indicate that you will listen to issues regarding that question, or hold extra 'office hours' - for hearing and addressing questions about the exam in a little more detail - once you are able to talk about the actual answer, and at more length. This will help you understand what it is people were finding difficult to understand or to solve. Also, this will likely have a positive psychological effect on the students - whether they did well or poorly.