A lot of people here are mentioning problem-solving. When you do problems, write them up in a way that will make for quick review later, and build good skills so that your problems on exams will be neat and clear (good for partial credit if your professor gives that)... and have a good chance of being "right."
I recommend the following direct tips on homework/problem-solving:
1) use a good writing tool. Keep a stock of them. Discard when unsatisfactory. Same with paper (be it lined, engineering-style (grid), pure white, the back of computer printouts if you're into recycling... just be sure you like what you're using and keep a stock).
2) do 1-2 problems per page to give yourself room to write notes or do small related calculations in the marginal areas. Use only one side of the paper, so the problems are quick to flip through.
3) rewrite the problem on your homework sheet (while tedious, it gives you time to focus, and it's MUCH easier to then use your homework later when you want to quickly review for a test).
4) retain units in your math (for physics numerical problems). Make to do the unit-math as well as the calculator math.
4) Check your answer for "reasonableness"... in terms of units and scale. If they asked for energy, and your answer has units of force, it's wrong. If they are looking for an "outer radius" and your answer is smaller than the "inner radius" given, it's wrong. If your car is traveling faster than the speed of light, it's wrong (unless you engineered something really exciting). Etc.
5) Box in your answer... Maybe write some notes about it nearby if it's interesting.
6) Throw out (or recycle) your page and rewrite if you made a major error or got sloppy.
With the prevalence of online homework submission, it's common for students to get lax and sloppy (about technique AND submission), but their performance on exams falters. Even if your professor uses an online system, be sure to personally use a good method of writing up your problems, and save the submission for a "break" if you need it.