I'm a huge fan of the Khan Academy and regularly watch his videos when I have a question about something in mathematics. Usually it only takes five or so minutes into the video for me to recall how to accomplish the task, and I can move along. I've always wanted to see if I could do the same thing for ECG interpretation.
So here is my inaugural attempt; rapid axis determination using leads I and aVF (assuming you're ok with a ±5° difference):