Prewashing can be a lot of effort though, since it's advised that you also leave the Nizoral in your hair for 3 - 5 mins as well. Especially if you do this in the mornings. Nizoral isn't THAT expensive that you have to do this anyway, is it?
I usually just give my head a good rinse in the bathtub (with that showerhead-on-a-hose thingie) and then work the nizoral into a lather - 1 to 2 tsp should be plenty, depending on how long your hair is.