If you possibly can, get a cubic D6 with pips, rather than numerals, on its faces. A D6 with properly arranged sides (1 opposite 6, 2 opposite 5, and 3 opposite 4) and pips (Dimples to indicate number instead of numerals) Is slightly more likely to roll high than low.
Why? Well, it may sound insignificant, but since there is only one hole on the 1st face of the die, usually the same size as the pips on the 6 side, there is slightly more plastic, and therefore more weight, on the 1st face than on the 6th face. This means that the center of gravity on the dice is not in the exact center, but slightly closer to the 1st face.
Knowing this, imagine that you, having thrown a die, have managed to get it to land exactly on the corner between the 1st face and either the 4th or 3rd face. It is more likely that the 1 side will be pulled down, all else being equal, and thus the result is more likely to be 6 than 5 or 4. Nice if you feel like messing with the bell curve. | 
