Critical thinking is a broad topic that cannot be defined quickly, though it can be defined succinctly. Some call it problem solving. Others consider it the process of deciding if an assumption is true, false, or partly true, and why. Still others simply say that critical thinking is thinking about thinking. But no matter how it is defined, in the age of the standardized test, most educators agree that critical thinking has been one of the first skills (or practices) to fall by the wayside, and students lack the ability to do it now, more than ever. Ironically, it is one of the most necessary skills for educational success. Some might argue it is necessary for life success. Even in answering this very question, you will utilize the skills involved in critical thinking.
That said, when it comes to teaching critical thinking, in my experience a teacher must maintain an open mind and understand that there are not necessarily correct answers. Communicating this idea to students is also paramount, and perhaps the most difficult obstacle to overcome. There should be expectations for continued growth, but I'm not sure that there should be an end-point in mind. Students want to be right. Students crave consistency. Building critical thinking skills is a process that does not necessarily lead to a specific or finite answer, which is very difficult for many students to grasp, and many teachers to teach.
There are a variety of practical ways to build critical thinking into any curriculum. In my experience, the two most important factors for success are time and student interest. If these are in place, with a little patience and often a lot of creativity, critical thinking skills can develop. I've found a lot of success using lateral thinking puzzles (or riddles) in my classes. These set a playful tone that get students participating, but the process easily translates to other areas of a subject when students are asking what an answer is and the teacher does not directly produce it.
Typically, the art of asking the right kinds of questions is the first step to guiding students through critical thinking. This means asking higher order questions, ones that do not lend themselves to one answer or simple answers. The Socratic Method is a very popular tool for developing critical thinking. Even this method can be practiced on a smaller scale by presenting open-ended questions that a class is likely to disagree on answers to. Debate is great way to build critical thinking skills, as it forces students to not only think about what they believe, but why, and then communicate effectively enough to prove it. Yet another critical thinking practice that can be incorporated into large or small parts of lessons is having students teach new ideas to each other. This forces students to think through a lesson from front to back, or to "think like a teacher."
Hopefully this information helps get you started, because even now, you could continue to further brainstorm ideas about critical thinking, as the subject is not only vast, but ever changing.