Make sure she answers the question that was asked and make sure units are correct, if relevant. Leave enough space for writing, and be reasonably neat. Cramming equations into insufficient space on the paper leads to errors. Try to develop a sense for whether the answer makes sense and is numerically reasonable. Always check answers if time permits. Do extra practice problems in addition to homework assignments if necessary to get completely comfortable with the material. If she is addicted to her calculator, it might help to go without on homework to help her develop mental math skills that might help her notice errors.