I usually say "an" if the h is silent. An herb. But if I pronounce the h then I use "a." A hospital. I think I picked up the habbit from Spanish class. For example, a feminine word that starts with "a" gets the masculine article because it is easier to say. You would say el agua, as apposed to la agua even though agua should be a feminine word. Another example would be el algebra. They just taste better that way.