when you've asked a question, you must then be quiet - do not interrupt - allow the other person time and freedom to answer
the other person (your 'prospect' in selling language) should be doing 80-99% of the talking during this stage of the sales discussion; if you are talking for a third or half of the time you are not asking the right sort of questions
do not jump onto an opportunity and start explaining how you can solve the problem until you have asked all your questions and gathered all the information you need (in any event you should never be seen to 'jump' onto any issue)
all the time try to find out the strategic issues affected or implicated by the product/service in question - these are where the ultimate decision-making and buying motives lie
if during the questioning you think of a new important question to ask note it down or you'll probably forget it
when you have all the information you need, acknowledge the fact and say thanks, then take a few moments to think about, discuss and summarise the key issues/requirements/priorities from your prospect's organizational (and personal if applicable) perspective