Since everything goes better with ASCII art, I feel that I should provide a nice diagram to emphasise some of the finer points of hole placement.

                                /
                               /  
                      ============
                      )          (
                      |          |
                      |   BEER   |
                      |          |
                      |          |
hole goes here  -->>   \         |
                      _/         |
                      \__________/

As for the hole itself, I used to be fond of a three slice technique where the first (and longest) cut was made along the length of the can, with two perpendicular cuts made along the bottom resulting in two angled flaps which, when pressed into the can, make an aproximately equilateral triangle. Alternately, you can make the cut along the width of the can first and then make the cut perpendicular to it, requiring only two cuts.

                                /
                               / 
                      ============
                      )          (
                      |          |
                      |   BEER   |
                      |          |
                      |          |
                      |          |
                      |     |    |
                      |    _|_   |
                      \__________/
Another popular technique among my friends was to have three angled holes comming together, resulting in three flaps.
                                /
                               /    
                      ============
                      )          (
                      |          |
                      |   BEER   |
                      |          |
                      |          |
                      |          |
                      |     |    |
                      |    /`\   |
                      \__________/
While the 3 flap method looks more impressive, the point here is to get loaded and not impress people with your surgeon-like blade handling abilities, and the 2 flap method it much simpler to do properly, particularly when you approach your desired level of intoxication.



Roninspoon mentioned that "I've always just used the can opener side of a bottle opener. No mess, no guff, perfect traingle hole every time.", but I wonder about getting a firm purchase on the bottom of the can with a can opener.