Some practical points:
- Spend time re-reading 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 confirmation/trivial clarification questions, briefly, but kindly and in a supportive rather than derogatory tone. (e.g. "Am I really supposed to do XYZ?" "Yes, those are the instructions."); don't try lecturing people about asking them, or telling the whole class again not to ask them. Instead, try to smile (if you can make it a non-creepy smile), 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 during the exam may uncover an actual problem with the exam which you have not anticipated! This has happened in not a small fraction of the courses I've TAed or taken.
- When getting a "how to 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 ok?" if you're actually concerned about them. Some people occasionally have "blackouts", or otherwise become very anxious, and this sometimes helps. This 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. That might sound irrational, childish, overly soft, immature to you - but remember you're testing the student's command of math, not your view of what their character may be like.
- 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 "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.