Magic Trick Crooked Dice

The most traditional way is to tamper with the dice itself, or get one that is skewed by design. There are two ways to tamper with dices, that I know of (others will point many different ideas, as I am no specialist).
One is to produce them with some kind of heavy material fit inside, slightly off center, to have them fall onto one side more often than the others. This is very effective, so much that it catches attention. It is also easy to detect, depending on the material you use. If it is steel, for instance, the dice will be attracted by magnets. Depending on how big the lump you include inside, the dice may get heavier than the normal (which is a tell-tale sign). To prevent this you should make the dice from a material that is lighter than the expected (a dice that looks like made from bone must have an interior made of something else) and carefully balance the lump inside. The combination of everything above makes the production of such dices very tricky and expensive.
The second method is to deform the dice (usually eroding it with sandpaper) before applying the dots. Such a dice will not be a perfect cube and will follow on the larger sides more often than on the smaller. The downside of this method is that people with keen eyes may be able to notice that there is a deformation of the shape.