After every test, I require students to fix all of their mistakes (regardless of how well they did). This form requires students to not only find a new answer, but to identify where they went wrong, so they hopefully won't make similar mistakes in the future.
I allow students to get help and use any resources they can for these corrections, and they count as a quiz grade. This also helps students improve their grade a bit if they didn't do well on the original assessment.
This works very well with math, but could also easily be used in other subjects.