Students often want to do problems by plugging numbers into formulas. It's often much more illuminating to first "walk" the student through the problem verbally. You want to encourage the student to think about the general problem-solving strategies needed for a particular problem. The student should then be asked to articulate the logic needed to go from the initial statement of the problem to the solution. Only after this has been done should you and the student start writing down the appropriate equations and formulas to work with. Also ask the student to question the reasonableness of the answer, the proper units for the answer, and the significance of the result for the situation at hand. If the student has trouble with the articulation aspect of this process, encourage the student to see the professor in office hours.