I get good results with clear, simple explanations. I apply that to my solution keys, which guide the student through answers step by step. I pay particular attention to the areas where many students have shown, time and again, they get tripped up.
For tests, the best ones are the ones with detailed solutions. Students often quickly learn by seeing the solution presented in clear steps.
I've also learned that for detailed solutions, students prefer hand written solutions. Time and again they tell me they have trouble pulling relevant information out of type written material.
So I've written up some detailed answer keys for many of the tests I use. When I haven't been able to do that, I at least provide answer keys.
Oh, and like Sal Kahn of Kahn academy, I'm a big fan of colour coding.