Copying % formula

Hi, I regularly calculate a series of number as a percentage of a sum that remains constant (expense item as a percentage of total expenses, for eg.) How can I copy the formula and keep the constant (eg total revenue). When I drag down, the cell number in that position in the formula lowers with the destination cell number. (Hope that is clear.) Right now I enter the formula for each cell I am calculating which is a pain. I am using Excel for Mac.
Elizabeth