A general rule of thumb is that people will answer specific questions with more detail than open ended questions or questions that imply potential for personal conflict (i.e. "how did I do" is unlikely to get honest answers except from your really closest friend - but instead something like "did you like the balance between roll playing and role playing?" or "what could we add to the table to make combat go more smoothly" etc.
i.e. ask specific questions that solicit input (which you can act upon)
Then after you act upon suggestions - especially relatively minor ones, it becomes easier to get answers (in the future) to questions about bigger items (i.e. after you fix little things about the scheduling of the game or the house rules or the layout of the table for combats then you can more likely get input about bigger items like game style, your DMing style, whether you should stop trying to do multiple voices etc.
Another suggestion - though this depends on the relationship(s) you have with players is to set aside time to talk with each player outside of the game session about their feedback. At the end of the night as people are packing up to leave may not be the best time to get detailed feedback - but grabbing coffee later in the week, chatting over Skype or even just some one-to-one emails or a quick phone call could get you far more detailed feedback.
You may also want to start the next session with a short (emphasis on short) group discussion about what is working/isn't working and quickly make adjustments that might help with the flow of the game - i.e. fix physical setup, talk about the time of the game, breaks, whatever.
With any game it is also important to keep in mind the expectations of the players and make sure that they match with each other and with your expectations as the DM.