One of the reasons students of math struggle at test time is that they fail to quickly identify "problem types". Let's say you're taking an Algebra exam and you see something of the form 4x2 + 8x -5 = 18 and are required to solve it. You should either be thinking about factoring the equation or if that doesn't work easily, using the quadratic formula. Typically, once a student identifies the problem type, he or she is 80% of the way there. Then it's usually just standard arithmetic (watch your sign changes + or - ).
Solving math problems is really a process in itself and involves: assessment, identifying the problem type, looking for other complexities, i.e. there may be several steps along the way, doing the actual arithmetic and finally checking your answer for logic. Does it make sense that Fred took 16 hours to reach Chicago from New York? If it doesn't, go back and look at your problem -- you probably missed something.
Be disciplined in your approach and you will excel in your math courses!