Preloading is the process of taking one drug some time (usually a few hours) before another to enhance the effects of the main drug.
Most preloading works by inhibiting or saturating an enzyme responsible for metabolizing a drug. Preloading can also work by giving two drugs with biological or subjective complementary effects.
Some preloading drugs actually offer protection from the toxic effects of the main drug, although many are really really dumb, dangerous combinations. I suppose I could detail some of these combinations here, but I don't want to be responsible for someone trying them, no matter how many times I yell disclaimers.