Sometimes a special rule makes you multiply a number or a die roll. As long as you're applying a single multiplier, multiply the number normally. When two or more multipliers apply to any abstract value (such as a modifier or a die roll), however, combine them into a single multiple, with each extra multiple adding 1 less than its value to the first multiple. Thus, a double (x2) and a double (x2) applied to the same number results in a triple (x3, because 2 + 1 = 3).
For example, Tordek, a high-level dwarf fighter, deals 1d8+6 points of damage with a warhammer. On a critical hit, a warhammer deals triple damage, so that's 3d8+18 damage for Tordek. A magic dwarven thrower warhammer deals double damage (2d8+12 for Tordek) when thrown. If Tordek scores a critical hit while throwing the dwarven thrower, his player rolls quadruple damage (4d8+24) be¬cause 3 + 1 = 4.
Another way to think of it is to convert the multiples into additions. Tordek's critical hit increases his damage by 2d8+12, and the dwarven thrower's doubling of damage increases his damage by 1d8+6, so both of them together increase his damage by 3d8+18 for a grand total of 4d8+24.
When applying multipliers to real-world values (such as weight or distance), normal rules of math apply instead. A creature whose size doubles (thus multiplying its weight by 8) and then is turned to stone (which would multiply its weight by a factor of roughly 3) now weighs about 24 times normal, not 10 times normal. Similarly, a blinded creature attempting to negotiate difficult terrain would count each square as 4 squares (doubling the cost twice, for a total multiplier of x4), rather than as 3 squares (adding 100% twice).