What you consider to be a personal question, and what I consider to be a personal question can be two entirely different things. There are some people who seem willing to discuss the most intimate aspects of their lives freely and openly, while there are other people who feel very uncomfortable letting others know of even the smallest personal detail. Most of us fall somewhere in the middle, and it's good practice to keep our private lives private, and only divulge the information that you feel comfortable giving.
There will, however, always be someone who out of curiosity will ask you questions that you find probing. If a person presents to you a question that to them seems like it's not significant, but it is to you, and you'd prefer not to answer (for example "How much did that cost you? Whether the thing they're referring to is a new shirt, car, or boat), a polite but nonetheless evasive answer is still "I do not seem to recall" or "It was a gift."
Either way, there is no need to get indignant or work yourself into a tizzy if someone's nosiness overpowers their sense of tact. If someone comes at you with a question that you feel poses a threat to your privacy, the best solution is to give them an answer, but only a part of the answer. For example, a friend of mine recently came back from a consultation with a divorce attorney.
A well-meaning friend who was unaware of the situation asked her how she maintains such a positive and happy marriage. Rather than break down in tears and let her situation get the best of her, she simply stated that "I think that it is important to be good friends in your marriage." This answer satisfied the question-asker, and she did not have to resort to lying, and she did not give away any of her secrets.
For those instances where you just have a rude jerk or jerk-ette and they are asking you questions trying to antagonize you and get you to react defensively you just have to ask them a question in return. What this will do is throw them off of their game, put them off guard and put them on the defensive. This usually defuses their offensive and makes them think twice about verbally attacking you in the future.
Many times single people are asked why they're not married. Usually they are asked this by married people in a smug manner. This can be countered with a simple "Well, are you proposing to me now?", Or a tad harsher "I am definitely flattered by your interest in my personal standing, but what really makes me wonder is why you're so interested in me? " If someone asks you if you're seeing a shrink ask them if they are recommending theirs. A jerk of a question deserves a jerk of an answer. If you're having trouble thinking of one in the moment you can just answer truthfully or refuse to answer at all as politely as you can.