Use the space provided on the same paper to avoid mistakes in copying the numbers.
Secondly, double check the numbers you are copying from the question, so that you don't do mistakes in interchanging the digit or missing out a digit, etc.
Thirdly, use multiple colour pens. First write with pencil, then when the space is finished, on pencil I used to write with green pen or blue pen and then red pen. It helped me differentiate between my previous written calculations.
Fourthly, double check your calculation there and then. If you leave it for later, you wont get time to check each and every one of your calculation.
Neatness helps a lot. Write clearly. Keep things in columns. Don't try to cram too much onto the page. Don't try to do too much in your head (i.e. write out each step). As you use terms to get to the next line, check them off or underline them.
For exams, though, go through the common calculations lots of times before an exam For instance, if you have a linear algebra exam, it is wise to compute matrix inverses a few times (say like 20-40 times depending on your brain's capacity to compute) to be at ease with the algorithmic details and be able to focus on the numbers more easily during computation.