Welcome to the Safety section of the webpage. Although most of the information seems very easy and obvious, it is still very important to remember that a simgle mistake can mean one less finger.
First off, always wear safety goggles when near someone working. Your eyes are the most important thing to protect during construction, since they are extremely important and very vurnerable.
Now, onto the more specific stuff:
Sawing
When you are going to saw something, it is extremely important to clamp the object down first. Make sure that you have used at least two clamps, and that the object can not move around at all.
Also, remember that there are many things that can go wrong with cutting metal, but they usually all spring from the same place: The hacksaw blade. If it is too lose on the hacksaw, then you won't be able to control the cutting well at all, making the result a piece of ..., and the hacksaw itself might get bent, ruining it for all future uses.
Here's a pathetic little game I made. You will need flash in order to run it, as usual. Just move the almuminum sheet and the clamps (they are magical, oval ones.) around in order to safely saw it in half.