As you may have noticed, whenever Excel generates a series from an initial entry, it automatically increases the series by an increment of one (be it by one day, one month, one hour, one minute, one widget, you name it). Fortunately, the AutoFill feature is very teachable so that you aren't stuck with always generating a series that increases the base value by one. As long as you provide at least two entries that exemplify how many units Excel is to increase (or decrease) the entries in the series, you can generate series with almost any kind of increments.
Well, what about a sequential series that decreases rather than increases (in other words, one that uses a negative increment)? To generate this kind of series with AutoFill, all you have to do is enter the larger value in the first cell of the range to be filled, the smaller value (the one that exemplifies the negative increment) in the next cell (either in the cell in the column to the right if you want to generate the series across the row or in the row below if you want to generate it down the column). Select both cells - the one with the entry showing the starting value and the next one showing the amount of decrease - and then drag the Fill handle to extend the series as far as you want to go.
For example, suppose I want to create a numerical series going down column A - starting in cell A2 with a value of 1000 - that decreases by 100 units in each cell below. To generate this decreasing series, you enter 1000 in cell A2 and 900 in cell A3. Then select the range A2:A3 before you drag the Fill handle down the rows of the column. Excel then enters 800 in cell A4, 700 in cell A5, 600 in cell A6, and so on. (Note that when you select the cells that demonstrate the increment to use, Excel knows right away that you want to generate a series, even when your entries are purely numerical.)