From the description, it sounds like what you're asking about is more along the lines of mental acuity rather than critical thinking per se (the latter is more

From the description, it sounds like what you're asking about is more along the lines of mental acuity rather than critical thinking per se (the latter is more about analysis in order to form judgments -- recognizing fallacies, rhetoric, and using deductive/inductive logic to form arguments). If you're asking about mental acuity so you can think faster and solve brain teasers at interviews, many of the other comments in this thread offer great suggestions. Achieving familiarity with the problem space and pure practice are keys. If you're asking about actual "critical thinking" (though I suspect that is not the case from your description, but it's an interesting topic that you might want to pursue anyway), the classic methods are the Socratic method, disputation and critical discussion of ideas with other people. If you don't have that luxury, you can train yourself by arguing with authors in the margins when you read books. Writing is key -- it helps crystallize thoughts and reveal weaknesses in thinking. Courses in philosophy also help provide a foundation on different approaches to reasoning.