Make a habit of writing your work in a neat and organized fashion. Sloppiness generates mistakes, particularly when a problem goes on for pages.
Always write equations. If you just manipulate expressions, you're likely to make mistakes when you plug them back into equations.
Always do the same thing to both sides of the equation. Don't subtract 2 from the right-hand side but divide by 2 on the left, or integrate the left with respect to x and the right with respect to y.
Start your homework well in advance of the due date.
Attend lecture and recitation regularly, and try to keep your mind focused on the material.
If you don't understand something, ask a question.
Come to office hours. Get some personal attention!