http://xkcd.com/759/ Alt text: "Handy exam trick: when you know the answer but not the correct derivation, derive blindly forward from the givens and backward from the answer, and join the chains once the equations start looking similar. Sometimes the graders don't notice the seam."
So, so, so true. I can really only remember getting away with this once, in number theory, but it was pretty spectacular. I wrote down what I was supposed to prove, started with something I knew, and worked forward and backward as far as possible.
1. Given
2-3. steps derived from given
4 ???
5-6. steps derived from statement to be proved
7. Statement to be proved.
It turned out step 4 was "Because 2 is an even number..." And my prof was of the school of thought that if it's that obvious, you don't need to write it down. 10 out of 10 points when I was only hoping for maybe 3 just for putting something down on paper.