The way I do it (which probably isn't the best) is to cut and paste the pivot table into another worksheet but paste as values only.
Then select all cells in the column you want to fill the blanks in. Then Edit, Goto, Special and click the Blanks option click OK. That should highlight all the blank cells you want filling. Then type + ,press the up arrow finally press Ctrl and Enter at the same time.
Probably havn't explained it that well but it does work really well. What you have done is highlighted all the blank cells then told the worksheet to fill the blank cell with the value above it.
Found this method via an internet search (which I can't find) so if this makes no sense Google is you friend.